I just started to build mine a couple of days ago and kicked around what engine to use. From doing some research , the OS .91 seems to be the choice , and would make a nice combo. The .75 AX would work just as well and may be a little stronger than the .91 but you`ll have more clean up. Both are good engines.
Maint, on four strokes is really not a big deal . Set the valves once a year . Some say they are harder to tune ,than a 2 stroke. I dont think their harder , just different.

If I had a Saito 1.00 , thats what I would go with , or the O.S .91 but don`t sell the OS .75 short , it`s a great engine .

Aquired a worn out Itailan ST .90 in a trade . Rebuilt it with new bearings, sleeve and ring , and the thing runs so well I`m going to stick that on my 4* .60 . Has a crunchy sound to it .
My 4* .40 with a Saito .56 would knife edge with lots of throttle and would run out of vertical or power at the top of loops , so I figured , what the heck , more power on the .60 wont hurt.