Snowmobile problems when im in powder!?

So i have a 1992 ski doo formula plus and when im on hard packed trail everything's fine but when i hit powder and the front end comes up the hole sled makes a thumping sound and this year i broke the drive chain and crank case so i need to replace that but should i replace my cogs also? and does anybody have a crankcase and sprockets for one? Thank you for your time!