OS91FS will give a little more power,theOS70 will give less than the 61FX,w8ye @ post 3 answered the pump question,if you don't need it, then it's just one more thing to go wrong,think you may have to use a longer mount,has been many years past since I've built the 4* planes,the other brands of engines I'm not familiar with,hope this helps.
Have a goodun,John