Re: siezed up seahorse 18hp
A normal top to bottom tune up for one of these guys is about $150, which includes a new carb kit, lower unit reseal kit, and ignition rebuild (w/new coils) and more. If your motor does not already have a fuel pump and you want to add on so you can run a modern, single line fuel pump set up, which most folks tend to do, that will be another $50-$60, less if you don't mind it looking less than stock, but that's an optional modification. As far as the internals of the engine go, that will depend on what it needs. You can get replacement cylinders and rings for about $100-$120 from places like ebay, but if you check vintage OMC enthusiast sites like aomci.org, one of the folks there may be able to hook you up rings, pistons and a fuel pump for quite a bit less. As far as paint jobs and cosmetics, that depends. If you want a showroom restoration, you'll probably need to have all of the components soda blasted down to raw metal (it's like sand blasting, but less harsh to the aluminum parts). It's probably about $100 to have everything taken down to bare metal, but it will save you TONS of work and give you a perfect finish surface to start with. Nymarine.com has factory correct colors and decals for you motor, but they ain't cheap. However, you can get colors that are very similar to stock from most autoparts stores for much less.
If you can get the motor to turn over and the internals are ok, it should only cost about $150-$200 to get her running again, including a decent paint job, but if you want to go all out and make it better than show room, you're looking at more like $400-$500, but it'll be the best lookin', best running motor on the water and will literally out live you. Hope that helps. Keep us posted. We love pics