Just bought an old Wheel Horse 416-8 for $200 for the engine. Will that work in my tractor? I'm thinkin it would from what I can see.Issue 1. Engine RPM at full throttle. The Onan is a 3600 RPM engine. The Kohler MAY be an 1800 RPM engine, depending upon the factory governor. You need to get that figured out first by using the SPEC number off the engine tag and talking to KOHLER,
Issue 2, The tractor frames of Onan powered Case tractors are cut-away to allow the engine to sit lower. Frames on the Kohler powered models do not have this cut-way. You will have to engineer a solution to that problem along with the next problem
Issue 3. Engine mounting. The Kohler engines are bolted to special mounting plates that in-turn are bolt to the sides of the frame using special rubber mounts. This calls for more engineering on your part. It is IMPORTANT that the centre line of the Kohler engine's crankshaft remains identical to that of the Onan's. Access to a Case 444 would be of great help.
Issue 4. Wiring. The Onan uses an alternator built into the flywheel. It appears that the Kohler has an alternator because it does not have a pulley mounted to the flywheel for a belt-driven starter/generator. And it must also have a starter motor because there is no pulley for pull-starting nor recoil starting. The 444 Case manual will show you how to wire the tractor once the Kohler is installed.
Issue 5. Crankshaft. The Kohler MUST sit in the tractor frame in the correct position in order for the PTO clutch to accept the drive belt for mower decks and snowblowers. Excessive belt wear or other belt problems will arise if the engine is not where it should be.
A simpler solution would be to find a used Onan from another 400 Series Case.