The other differences.
4 strokes need to be bigger than 2 strokes for the same application.
Example: That 61 you have would be about equal to a 40 size 2 stroke and should be used in a 40 size plane.
2 strokes run at higher RPMs and are great for speed.
4 strokes have higher torque and are good for aerobatics and climbing.
4 stroke require you to adjust the valve lash from time to time. It is not hard to do and not required often. I set mine during the winter downtime.