I'm sure you could do it. I converted a 38" span PT 19 from Hobby Lobby. I just glued in a piece of plywood for a firewall. That plane was kind of heavy though and would even fly on the .061 PAW I bought for it so I yanked it out and put in a .09 PAW but at almost 30 ounces it flew like a brick. I flew it about 30 flights before I crashed it. That engine still wasn't broken in and it wouldn't idle for long. PAW's must have to run forever to get broken in.

The plane will get heavier after you modify the fire wall and fuel proof everything.

I mounted my engine upside down but I'm not sure they will idle well upside down. Of coures that is what looks best on a lot of scale planes and might look good on the SE5.