It sounds so far as if you would do best by fitting the entire 12 hp engine, including starter and charging system. Since it was made in 1991, compared with 1984 for your current 8 hp engine, there is a good chance it has quite a lot of life left in it and it is probably a slightly more modern design. I do not recommend using a starter without a proper charging system (as on the 12 hp engine) as it usually ends in poor battery life, and at best it is an extra chore, remembering to charge the battery regularly.
Does the 12 hp actually fit into your mower? Can you post some pictures of the two engines - maybe installed in the mower?
