Tom- If it's the same as the 165hp mercruiser then it's a chevy/GM 250
cubic inch inline or straight six, very popular in the seventies, but
I've never heard of a 200hp version, I think the 200hp would be the
292cu. in. chevy/GMC I'm not sure if the balancer would be the same, but
you could check in the local auto parts. The 292 came in chevy/GMC vans
from 75' thru 78', probably came in trucks too. Good Luck. Tom.