Re: Good Replacement Engine
first you need to verify that the reason the engine won't turn is the engine - a seized upper gearcase in the outdrive, or a failed gimbal brg may cause the same symptom - remove the outdrive and retry turning the engine manually
then, if the engine is not useable, yes you can use a Chev 305 or 350 long block to replace it. Just get a truck 305 or 350 from a similar year (1988-1991), remove tin core plugs and replace with brass. You can also replace the original head gaskets with a composite type with stainless fire rings - e.g. FelPro (optional, but required in salt or brackish water)
If it's a truck 350 you can usually get away with the stock cam, but if auto, or unsure, install a marine camshaft.
Then, use all of you marine bolt-ons- electrical, fuel system etc.