You have opened up a can of worms here. The choice between two stroke and 4 stroke is normally a matter of opinion. I have both types in 3D aircraft. I have a Reactor as well and I use a Saito 82 in it. I know there are plenty of people who use the 55AX in the Reactor with no problems at all. I would not reccomend mounting it up-right either. I believe the plane have you mounting it at a 135 Degree angle so the muffler will go out the bottom. You will get alot of opinions about 2 and 4 stroke engine for 3D but you need to see what work best for you especially since engine availability is limited for you.