I bought a 30 hour mechanic's special 2004 1800 SR3 with a 4.3 liter Mercruiser with a cracked block from freezing. A naked longblock from these guys with high compression pistons left me shy of $2,000. https://www.rapidomarine.com/default...ngines262.aspx
All the original ancillaries were perfect, so I swapped them all over. I used a forklift at our work yard to remove the old one/ install the new one. Didn't even have a helper, unless you count beer as a helper. And it IS. Swapping over all the ancillaries was more time consuming than you would think, at least for me. I chased threads with a set of taps, and generally took my time. Your local Autozone will have an oil pump priming tool for free rental, even if the kid at the front desk doesn't know what it is. DO NOT skip this step.
I've got about 12 hours on the new engine. So far so good. Their website could use a little help, but they offer fully assembled engines as well. I enjoy this type of stuff, but it's quite the undertaking. If you find a good mechanic to do the work at a decent price, write the check.