I know this is an older thread but there are some interesting questions brought up. As far as the PTO
goes, The one in my Bolens can be engaged with the flip of a lever. Uses the same 2 belt pulley's and the same tensioner. The pulley on the crank and the pulley on the PTO
lined up with no shimming. With just a little planning things can be made to work.
As far as the cost of the engines goes, I have yet to pay over $475.00 for an engine. Water or air cooled.
The original engine in the Bolens was a 20hp kohler twin cylinder gas engine. I installed a 16hp Kubota twin cylinder diesel. This is rated I believe at 3600 rpm. I'm running a max of 2880 rpm.. I know for a fact that that this diesel will do 50% more work than the gas on 1/2 the fuel. I have yet to stall the diesel with chains on the wheels and the gas engine could be done without a lot of effort without chains under the same circumstances.
Next is the 10hp Wheel Horse. I installed a Lister-Petters 8hp diesel. This was rated at 3000 rpm and I'm running it at 1500. According to Lister it should be putting out 5hp. at this rpm. With the tractor in high range 2nd gear it was impossible to stall the engine. This with over sized new ATV tires. I'm willing to bet that if you took a 15hp gas engine at the same rpm and tried the same stunt the engine would have stalled instantly. Just as a final note. the 5hp diesel ended up destroying the differential on the Wheel Horse.
Horse Power has been a huge sales item for the manufacturers. The truth is that horse power without usable torque is worthless. Torque moves the world.
I don't believe it is practical for all operations such as outboard motors, aircraft, small lawn mowers and other light weight applications but even that is changing. I was not a big follower of diesels until I moved into farm country. After operating diesel and gas tractors side by side it soon became apparent that diesels had a sizable advantage over gas tractors in the field. Trucks, earth moving equipment, ships and the military aren't using diesel because they like spending money on expensive engines. It's because diesel gets more done for less money than gasoline. The rest of the world as well as our own rail companies realized this years ago.