I have Saito 100 on mine(46),no problems balancing with 23oz engine. The fuse is long and allows a large range for the CG. I dont see another 3 oz. as being a huge problem,I have cut out the fuse bottom and mounted the battery/rcv just behind the wing saddle on the 60 Do, using a hatch I made. If you are not worried about making some mods and overpowering it,go for it,otherwise,put the 50 in it.
As for the 2 vs. 4 stroke thing, I've been there and back. I went to all 4-strokes,then realized that there is no special advantage to them. You can achieve the same thing with a 2 stroker and save money on engine,fuel,repairs,props. Just use a powerful enough engine to do the job.