The 80 4 stroke is a very heavy lump iit's very large also!
We run one on the back of a Gemini 5.05 (75hp same block) and I hate it the balance is all wrong. Having said that it's particularly frugal and very reliable and gave the previous owners many happy years of service.
We also run a 100 merc 4 stroke (same block again) on the back of an XS 550 and that is a little better but definitely is arse heavy
You will undoubtedly have more fun with the Opti but I suspect with about a 15% fuel uplift.
I kinda thing Dais suggestion of an Etec makes sense but then us Welsh ribnobbers tend to stick together!
Here it comes again, I don't stand a chance
Soul possession, Got me in a trance
Pullin' me back to you - Deja Voodoo