I've had propane powered trucks and run propane powered tractors. They do run clean and have a long life span.
As far as power and torque on the older tractors the gas and diesels both had good torque curves. Most of the factory propane tractors had higher compression from the factory to produce the same power as the gasoline versions. Those engines were low speed engines at the time many maxed out at 1400 to 1800 rpm from the factory. They did not start good when you got to the sub zero temperatures. Once started and warmed up they were fine, they used a water heated vaporizor to turn the liquid propane to a vapor for the carb.
The larger trucks and pickups also used a liquid heated vaporizer, some had a vapor bypass to get started when cold.
Years ago we refueled with "thief hoses", a simple hose from the fill tank to the tractor tank, open the fill valve and start venting vapors from the tractor tank till it started spitting liquid from the vent, shut the valves then as she was full.
You will not find any new tractors using propane as there are no new gasoline powered tractors.