yea, I have an '81 25hp. I found the starter/bracket on ebay. If you want to charge your battery when running, then you'll need to buy a complete electric starter kit or find a parts motor with electric start.
I think you'll find that it will cost more to do it than it will be worth to you. I checked on one for my 9.9 johnson and it was going to cost more to do it than i paid for the motor