Can a regular gas engine i.e. truck engine be used in place of the cletrac engine on these tractors???
If you've got the time and money to make the necessary new connections, almost certainly; you just need an engine that fits, and fabricate...
But whether it's a good idea depends on what you're trying to accomplish: just get it mobile?? run it occasionally in demos or parades?? actually work it, a little or a lot??
Truck engines can be as different from tractor engines as truck engines are from car engines:
Car: deliver smooth power over wide range of engine speeds thru 3 gears (not counting more recent 4-6 speeds for smaller engines).
Truck: develop smooth power thru narrower rpm range, using more gears as much as rpms to vary speeds.
Tractor, esp crawler: develop smooth power thru generally narrowest rpm range, often designed to develop max power/torque at lower engine rpms than car or truck.
Putting a modern higher rpm engine in might overstrain components built for lower rpms and/or overstrain the engine running it slower to match the tractor gearing ;what was it's original engines rpm designation in the 20's?
Hope I didn't cloud up your day!!