You should have no problem finding a model Homelite engine like I posted. They are a dime a dozen. look at yard sales and on Ebay repair shops. They are real cheap. I have piked up a complete triummers for $10 just for the engine parts. I have lots most are the 30cc models with clutches. Your pic showed a homelite in the front so I figured you had one. Here's are some mounting pics. Homelites are easy to find parts for. As for the 3 point mount you can get these parts at a hardware store in the plumming department. You will need. a 1" pvc adapter and a 1" to 3/8" brass reducer and a 3/8" compression washer with a nut and a 3/8' semi flexable pipe. The pvc adapter will need to be turned dow to7/8" to fit the front housing and you need to get a teflon tube at the hobby store. Brand Octura 3/8" teflon tube liner. If you have the flex shaft from the trimmer you can use that as the flex cable. Otherwise you will have to get that too.
I do all my own modifying to the engines I have. So making parts is no problem for me. I have Zenoah engines too but not for boats. I don't trust them yet. I have lots of tuning problems with them. The manual says 25:1 oil mix but I can't get them to run. I am now at 40:1 with synthetic oil and they run better.
Homelites are so easy to run and bullet proof. I have sunk them several times and they start right back up.