If you already have the MDS 78, and it runs well, it should have plenty of power to make a fun plane. I've read good & bad about MDS but have never actually seen one on a plane.

A 91 four stroke (any brand), or the Saito 100 or 125, should be good too.

All of the 60 size sticks (any brand) I have ever seen have been running four strokes. I have an OS 91 on a Big Stik 60 and it has plenty of performance.