3W engines are pretty nice from what I understand, I've considered them before. I like parts support and availability so I tend to stick with brands like OS, Saito and DLE for the most part. If I see something else I gotta have then I have to really see if there's other choices in engines and weigh the pros and cons. I tend to look at the specs for compact light weight and highest power in most engine types because I fly mostly warbirds and hate engines sticking out of the cowl.
So Roto is very interesting to me because they got the head design right. It's been designed in Solidworks and the flow of the head has been studied using computer fluid dynamic analysis. The intake is opposite the exhaust like full scale engines for one. So for your FW take a look at the big twin they offer, 85cc I think, From what I remember it should fit in the cowl but take a look at the drawings of the engine for the overall measurements. If you printed them out at 1:1 scale you can place the cut out of the engine in the nose to see what's going on.
I have a SiST FW kit I'm waiting to build and I'm seriously considering the Roto engine over the Saito twin gaser.