I displayed next to Bob Manske in LeSueurr Minn one year. I think I rememeber a story about Bob had built three of these tractors. They were for the fellow who owned the Caterpillat franchise somewhere and his sons.
What are the difficult parts to do on the Holt engine? I know the valves may be a trick!!! Thanks Capt,nYou can see pictures of the engine and castings at "Cole's Power Models" web site. They sell the drawings and casting kit.
It is difficult to build. If you would like to build a 4 that is not as difficult, Cole's also sells the E.A. Wall 4cyl. L head and OHV 4cyl. and also the Challenger L head V8.
Any of these engines will keep you busy for a long long time.