Re: Mercruiser 2.5L
Depending on the year if your 3.0.
I swapped a 2007 3.0 into my boat that had a 1970 vintage 140 and outdrive.
I had to use the new updated coupler and a different flywheel to reuse my starter.
Two different flywheel sizes. I think a 12 3/4 and like a 14".
I was also told the heads can be swapped. My old 140 head has the large ports, the 2007 has the small ports.
Reusing the manifold took a special metal gasket to couple the old manifold to the small port head.