The MM 403 4cyl engine is tough to beat. It bores to 436. I have a cheap basically stock .060 over 403 making 100 ponies. Only real mod was the cam. I pull 5500# 4&5mph in a UB (not stock classes). It usually places but there's plenty of room to unlock more ponies. 85 is very reasonable. The G can balance ok in 6500 & 7500.
Not to knock a Cockshutt. A big Herc would be fun.