the 77 maico is a nice bike (a bit rare and hard too find)
back in the day the rm370 won nearly everything
and yamaha 400d wasnt far behind
with all the modern technology we have to improve these bikes (suspension pipes ignitions etc etc )
then any one would be a good choice
i have raced a rm370 and a yz400d
both good bikes but i would lean towards the rm (not cause i own one)
kawasaki stopped making kx400 in 76 it wasnt in the same race with the above bikes
special mention husky 390 very nice handling bit of gear