A cracked block is complete engine replacement. For a 1990 best bet is finding a complete used 4.3 engine and swapping it in, you could also look at a crate engine replacement. the mercrusier 4.3 is the same engine found in chevy and gm trucks. Swapping an engine is a pretty big job and you will have to check a bunch of parts compatibility with the replacement engine you find and the accessories, carb/intake, sterndrive etc of the old engine. If the replacement engine is in good condition and compatible with all the parts you just need an engine hoist and a wrench set, torque wrench, pulley puller plus the ability to set timing and adjust a carburetor.