I know of a couple of Mainship I repowers with 6 cyl Yanmars, but I don't know what engine models they were. If you are askking if it will fit, I would say yes for sure. The boat has a 5 degree shaft angle which you can measure for yourself. You may end up having a nose down engine, but that's fine if done correctly, many/most of the old Mainship repowers end up nose down.
Depending on horsepower you may need to upsize fuel lines, exhaust, and water intake and prop shaft. I assume you're replacing an 8.2 Detroit?
I did a repower of a 78 model I which is the same hull. I used a Cummins 6BTA and that fit no problem, but everything I mentioned had to be upsized.