If the old engine is raw water cooled don't even think about a rebuild. The old engine, even if rebuilt is just waiting to fail due to something inside corroding thru.
I don't know about all the manufacturers but when I repowered with a new Beta they made engine mounts that fit onto my old engine bed. I just had some adapter plates made so that the new mount bolts and the old bolt holes worked. No big deal and definitely no rebuilding the engine mounts!
Also, I did all of the installation work on my project and I'm just a back yard mechanic. The two things that were a bit unusual were that the new engine turned opposite of the old and the throttle hookup was also backwards. I now run my transmission in reverse for forward, no big deal for a Volvo Sail Drive, and hooked the throttle up backwards also. Both readily achievable.
The bottom line. Don't give up on a new engine of a different make until you have looked into it very carefully and are sure it just won't work.
Current engine is a fresh water cooled engine with a heat exchanger.