I put a 105 combine engine in my 4020 about 15 years ago. Excellent running 2500 hr engine for $800. It has served me well since adding a powerful electronic ignition and getting rid of that rerdiculous oversize stock carb and replaced it with a smaller one from a 283 Minni Mo. No more smoke,Hard starting, fouled plugs or spitting, uses less fuel and much better low to mid rpm performance.
They also crank much better if you replace the way undersize 10MT starter with a twice as powerful 20MT.
Those engines are harder to find now, too old for most salvage yards and ones parked in the tree row for 10 years are mostly rusted up.