Lots of people got on the super-cheap K10 fork bandwagon a few years ago. They are a great fork for being in the sub $100 range. Several write-ups exist on how to add disk brake mounts. There are places online that detail disassembly and reassembly so I won't cover that here. It has been said that you can't shorten K10 forks. You can do it with the right tools rather easily. I'll show how I shortened mine 1 inch to fit a Garelli Rally Sport moped.
Step 1: Examine the Fork Tube Construction
Look at the tops of your fork tubes (after you pull out the little rubber grommet). You will see that a threaded bung has been plug-welded in. To shorten the forks you need to cut these bungs out so that you can re-use them later.
Step 2: Measure Bung Depth
First step is to measure how deep the bung is welded in. You will need this measurement later. My bungs were 7.16mm down from the top. The accuracy of this measurement isn't critical, but try to record it within 1mm.
Step 3: Bung Removal
On the lathe, zero your tool on the end of the fork tube.
Slowly remove material from the tube until you get to the diameter of the bung.
It will fall out. Don't loose it!
Finish cutting the tube down to the length you want it to be.
Step 4: Re-examine Your Tubes
Looking at the end of the tube you just cut, you will notice the inner diameter of the tube is too small to fit the bung back in. You will have to counter-bore the tube to fit the bung.
Measure the thickness of the bung (8.13mm). You'll need this measurement in the next step.
Step 5: Counter-bore Bung
Back at the lathe, zero your boring tool to the end of the tube. I like to use end-mills for boring. None of the vibration and chatter that is associated with a traditional boring bar.
Remember that first measurement you took (7.16mm)? Add that to the bung thickness you just measured (8.13mm). This is how deep your c-bore needs to be (15.29mm).
You could measure the diameter of the bung to determine how wide to bore. I just put the tube I hadn't cut yet into the lathe and zeroed my tool on the inner diameter.
Step 6: Drill Weld Hole
On a milling machine clamp the fork tube in a v-block.
Choose a tool approximately the same diameter as your bung is thick.
Zero your tool on the end of the tube.
Move the tool the distance of your c-bore.
Punch a hole all the way through the tube.
Step 7: Ready for Welding
Now you have two tubes cut to the new length and two bungs ready to be welded in place.
Step 8: Welding Bungs
Push the bung all the way to the bottom of the c-bore and give each hole a tack-weld.
Then go back and fill the holes with weld bead.
Step 9: Cleaning Welds
At the lathe again, machine down any protruding weld.