So I've decided that I'm going to buy a lower unit and put some sort of motorcycle or small 4 cyl engine on it. What would be the best lower unit/frame to do this with? I'm hoping for somewhere in the 40-60hp range. What can handle that as is with few mods?

If your going to buy a lower unit and then go through the trouble of trying to custom fabricate something so it all works only to accommodate in the 40-60 up range, why don't you buy an entire unit and have it modded? A stage 3 is around 52-55 hp. To answer your question though, in the past one guy used an engine from a Kawasaki motorcycle on a gatortail lower unit. I believe it was in the 1200-1500 cc range, he had problems with the electric clutch holding up but it did work. IMO the best lower unit would be the BOSS due to it not having an electric clutch and the fact they already have the boss running on them which is 75hp. A machine shop strapped a motus engine to a boss lower and had some impressive results.

Short answer: $$

I figure I could find a lower unit for under a grand, and I have some buddies with all sorts of bike engines laying around that I can get for cheap. Whereas a new modded unit is going to cost upwards of $5k easy. Along with the fact that I've seen guys tearing up highly modded motors because they just werent designed with that much power in mind. So even though they work, and do put out a lot of power, they just dont have the reliability once you put a lot of hours on them.

I would love to have more than 60 horses running through it, I just didnt know what unit could actually take that much power before tearing something up consistently, like the electric clutch you mentioned. The boss unit would be perfect, but I didnt think finding just a lower unit on such a new motor would be easy. But I'll keep looking. I'd like to have something with reverse as part of the outdrive.

I havent looked at the Boss 75 in that much detail, if they arent running an electric clutch, what are they running?

Thanks for the input.