Electric Motor Tricycle 6V4.5Ah

I bought a riding toy for my son and it's cool, but it doesn't have enough power sometimes.  It's fast enough in the house, but it gets stuck outside.  The plastic wheels tend to slip on rough surfaces and inclines, but I can address that issue at the wheels/tires to some degree.  However, the battery that came with it is a 6V4.5AH/20HR sealed lead-acid rechargeable battery (It has F1 contacts, so unless I change or adapt the wires/connectors, I think I need to stick with a battery that has these).  The AC-DC adapter has 6V DC 500mA output, but I think I could get a UNIVERSAL AC DC ADAPTER {1.5V 3V 4.5V 6V 9V 12V 500mA} that would work with a new battery.

I'm hoping that I can increase the power noticeably but safely.  One constraint is the width of the battery area inside the "engine" compartment.  It fits the present battery (1.75 x 2.75 x 4), but I'll have to mod the compartment to fit a replacement that's more than slightly bigger.

I've seen some 12V4.5AH batteries online, but most are larger in every dimension.

Am I even on the right track thinking a higher voltage battery is what I need? 
Could I use an NiMH or Li battery/batteries instead?

What's the easiest, safest, cheapest route for this mod?

Thanks,
Dan

kdf.inc19694 years ago
dont know about the battery part but as far as the plastic wheels slipping outside have you tried wrapping something like an old bicycle or lawnmower tire around the wheels to give them tread? you would have to cut the side off i think and just use the flat middle part but should give nice traction. havnt seen it done yet but i have seen old bicycle tires used for better traction just never on a riding toy.