How about an old MB reefer engine? There should still be some of those around and they should already be set up to run at a constant speed. The main problems I see with this over an auto engine is availability of parts and if it is powerful enough.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k