Kitty Cat Snowmobiles

I bought a '96 Kitty Cat for my 4 year old boy. He weighs about 50 lbs and our yard is sloped. Its bone stock. When starting, it boggs down, like a chainsaw thats stuck, and I have to give him a push to get him going. I've read about clutching, sprockets, tying the govenor and deep snow kits. Could someone give me some direction as to what would be the best bang for my buck? High speed is not a priority, but general driveability in some moderate snow is.

is it an engine bog or bog because something is not right in the drivetrain

you should lift up the sled and check every roller underneath and make sure they all turn free, if not you may need to pull out the shaft clean and lube the wheels, this happens a lot on these machines and i am certain if you have to push it to get it going this is usually the problem, also check your brake band and brake cable make sure thats not too tight or sticking
there also 2 bearings on the drive shaft you make sure spin easy

if its an engine issue are using good 2 stroke oil??? synthetic?