Bottom Engine Assembly MESSED up...

Bad deal on that, sorry. Once these things run loose a while they tend to wear a pattern in the shaft, and/or gear, and want to return to the worn spot. IF you have not too much play where these parts are supposed to ride, you can realign and use Loctite sleeve retainer on the shaft and pins, then, put Loctite 262 to secure fasteners. If you have a minor wear pattern, you could build it up with JB weld and file to size, again use 262 on fasteners. I have no experience with this engine, but these are general procedures that may save you from trashing the engine, and get a little satisfaction turning a bad experience into useful knowledge. I once saved a Honda camshaft using JB weld. The car was not worth the price of a new cam, so we made do, It still runs! Best of luck to you.

whatever that guy said above, ignore it. call the place you got your engine from and return it. every good engine retailer offers at least a 30 day return. take advantage of it...

I'd pull both the clutch & drive gear to check that they are both seated on the taper AND have the woodruff key in place - seen a lot of new engines in which one of those keys was jambed in behind rather than in the slot

also, look for a crack in the drive gear right above the key slot

Sorry, bairdco, I was making an assumption he was OUT of warranty. Maybe, maybe not. When out, there are many solutions,and opportunities. Speaking from 52 years as a master powertrain tech. Thanks

your warranty is your best friend when this happens trying to get a replacement is the best option