My son wanted an MGP scooter and at the time they were ridiculously expensive. So I decided to make him one instead...

Step 1: Assemble the Following Ingredients

Parts

A bar stem from a mountain bike.
A section of angle iron (I got mine from an old section of bedframe).
A handlebar (Straight, riser, whatever you have).
A razor scooter fork (you may want to substitute this for an aftermarket fork that is stronger).
A few lengths of flat steel bar 25mm wide.
A section of square steel tubing to connect the deck to the head tube.
A mountain bike headset that fits the fork steerer.
A section of tube for the head tube (I needed to buy mine in the end as I didn't have anything suitable in the scrap bin - it ended up costing $0.80 so no biggie).
Two inline skate wheels or scooter wheels with bearings and axle bolts.
A length of steel tube from an old bike frame (this needs to be narrow enough to slide inside the end of the bar stem).
Another section the same inside diameter as the outside diameter of your forks steerer (and able to slide inside the bar extension tube listed above).
Two head set clamps from old razor scooters.
A rattle can of rust protecting undercoat.
A small tin of hammerite paint.
Offcuts of grip tape (Skate shops will give you these free of charge).

Tools

Angle grinder with cutting and grinding disks.
An arc or mig welding set (beg, borrow or steal one if you don't have one).
A sharpie for marking out.
A tube of superglue (handy for temporarily holding things together while you tack weld them).
Clamps (magnetic and normal).
A tape measure and set square.
Calipers (Optional - useful for matching tubing to various parts).
Dremel with cut off disc (optional).
Electric drill or drill press and an assortment of metal cutting drill bits.

Step 2: Top of the Handlebar

The first thing I did was fabricate the handlebar.

Take your mountain bike bar stem and match it to the length of frame tubing you have. The length of the bar is up to you - I made mine to the length that my son found most comfortable.
Mark on it where you will make the cut and whip it off with the angle grinder.
Grind the paint off the end of the bar so that you can weld it.
Press fit the two parts together to check the snugness

Step 3: Bottom End of the Handlebar

Find a section of tubing that fits over the steerer tube of the fork you have selected. This is sleeved over the top of the bar tube.
Cut a mitre in one end of the sleeve .This makes for a stronger join when the two pieces are welded.
Drill a hole in the other end (line it up with the flutes on the other end so that it aligns nicely).
Mark a line from the hole to the edge of the tube making sure it is parallel to the long axis of the tube).
Cut the line with a dremel tool or an angle grinder with a super thin cutting disc).
Fit all the parts together and make sure they are perfectly aligned.
Weld 'em up so they'll never be parted again.

Step 4: Fabricate the Deck

The deck is made from angle steel from a bed frame and some pieces of flat bar.

Start by cutting two lengths of angle iron to 500mm each.
Mark a 45deg angle bevels to both ends of each deck section.
Cut with an angle grinder.
Clamp the two ends together and drill a hole through both sections the same diameter as the axle bolts. You'll need to test the location before you drill to ensure there is enough ground clearance when the rear wheel is in place.
Cut two lengths of flat bar to go between the angle sections.
Prep the surfaces to be welded by giving them a tickle with the angle grinder.
Clamp the parts together making sure the bolt holes are aligned (bolting the wheel in temporarily is a good idea)
Tack weld the sections in place.
Weld the entire length of the sections.
Grind the proud bits back.

Step 5: Fabricate the Head Tube and Stem

This is the most delicate bit and requires some patience and trial and error to get it absolutely lined up.

Cut a rounded bevel in the end of the square section. Match this to the diameter of the head tube you have selected. I cut a straight bevel first and then teased out the curve with the angle grinder. Keep grinding until the parts align as tightly as possible and as straight as possible.
Check that the bevel angle and length of tube is correct before you make the next cut. You are aiming for the deck to be level when the front and rear wheels are in place so assemble the fork, wheel and headset now to double check.
Cut the bevel and align on the deck making sure it is set up square and flush.
Use a magnetic clamp or superglue to hold the stem to the deck and then tack weld.
Weld that puppy up.
Clamp the head tube on and weld that too.
Get on that angle grinder and give those welds a good one-two to smooth them out.
Done!

Step 6: Stick It All Together

Now the best bit - seeing if it all goes together...

Bolt on the rear wheel.
Fit the head set and fork and grease the bearings well.
Fit the front wheel.
Clamp the handle bar to the stem.
Clamp the whole bar assembly to the steerer using the two head clamps.
You now have a functionally complete scooter!
If you want to paint your new ride go ahead and mask off any surfaces you don't want painted and go to town with the rattle can of rust proof primer
Once that's done give it a good lick of Hammerite (for durability).
Stick on your griptape offcuts and you're done!

This got two thumbs up from the boy so I consider it a success!

Note: This scooter does not have a brake! I discussed this at length with my son and we came to the conclusion that a brake was not required. You may decide otherwise, in which case, you'll have to figure it out yourself and post the results :-P