Cart losing power as it warms up

[fixitman5453]

I have an old Melex cart (no idea what year or model) that was given to me.
The engine is a Vanguard 1cyl OHV 4-cycle. I'm guessing a 9HP. The engine is labeled "Vanguard made for Columbia ParCar by Briggs&Stratton". I cannot find a part number for the engine.

This cart used to run fine until it was taken for a joyride by some local teenagers and rolled in a cornfield.

Now, when the engine is cold, it runs like a raped ape. As it warms up though, it gradually loses power until eventually it is barely running (not enough rpm's to engage the clutch). If left to sit for an hour or 2 it will run again. Each time this happens though it is less "apelike" the next time it is used.

I have been told by a local golf cart guru that I probably needed to rebuild the engine. He suspects bad rings.

I's hate to go thru the trouble and expense to rebuild this engine just to find out it was something else.

Any suggestions on other things that might cause this that I can check and maybe rule out?

I am planning on checking the compression cold and then again when hot to see if there is any major difference, and if so trying a tsp of oil in the cyl to see if it goes back up. (I guess I do remember something from working on those old Chevy's )

Any suggestions appreciated