1976 233 Merc starter question

[blittle913]

I just bought a 1976 srv with a 233 merc.. I suspect the starter is bad. The boat will crank over but it seems stressed, and as soon as the boat wants to fire once the starter will kick out. This makes it extremely hard to get the boat to start with the starter disengaging every time. I pulled the starter and got it tested and it tested good however I think the drive is weak anyway. I started searching for one online now and everything I find has an attached solenoid. Mine does not. Mine is just like a ford vehicle with a solenoid similar to the ones mounted on a firewall. Why cant I find one like this? Thanks for any replies!