Sounds really old school, but in the early 90's I built 3 250 yamaha mtn sleds for my son and nephews. Used to be a thread on snowest just for these modded sleds.
I still have the most beatup one at my cabin ( the road warrior covered with stickers ) and use it like a motorized skakeboard. The biggest issue is the stock clutch's are built right on the end of the crank , so first issues is pulling crank and machining a taper to run phazer clutches.
For several years we had fun with mods: 32mm flat slide Lectrons, PHazer struts, 121 tracks with paddles, Phazer primary clutch's, taller seats, utility racks, found gas tanks off the later 300 so tank was on the tunnel.
Two weeks ago with grand nephews at the cabin, no question the go to sled for 10 year olds. Shocking the powder it will go through with a 90lb rider. The M5 ok, but with power and the ability to manhandle a sled in the powder, the line was waiting for the 250.
A year ago one of my snowbiking friends was looking for a kids sled. I mentioned the old school Yamahas, then he found one, his daughter and two 9 and 10 year old boys wore that thing out in a winter. They live on 20 acres a ways out of town, we had valley snows, after school until bedtime the poor old thing was being ravaged. Good way to enjoy the outdoors.