well u can start with the basic paper clip and a zip tie.
after that it all cost money.
best bang for the buck is to put a brigs animal in it.
yeah it will cost u a few bucks but your old motor will sell for $100 to 125.
and U would have to spend thousands to come close to the power of a animal.
and if the kid wants to race there is a class for the animal /lo20 and less cheating going on because of the hologram cable sealed case .
the best mod for the buck is spent in the junk rear skid link to mod
http://www.hardcoresledder.com/forums/350-mini-sled-forum/97822-polaris-120-suspension.html
I have done this to a few of my buds kids 120 and it is funny to watch a mod skid run next to non mod skid polo stick
http://www.recmotor.com/products/briggs-stratton-local-206-semi-pro-pro-120-class/
with a brigs lo20 the 120 will do 35 all day.
the cheapest option is to buy one already done
I may know of 1 in MN a poo but I see you are IL not worth it.
deff guys out there who spent the time and money and now the kids are already on to the skidoo 300 freestyle class