I don't know 1 person yet that had to add any dead weight (lead) to the Sundowner 50 when using a 2 stoke 61 or 91 to get the correct CG. Feel free to look in the SD thread in the extreme prop section and you will find many there using an OS 91 including myself and did not have to and any dead weight to get the correct CG. We just moved our battery around to get the correct CG. I do not mean to put anyone down, but if you have been in this hobby long, we learn how to move things around to get the correct CG when going with a bigger motor.
ORIGINAL: bigedmustafa
The FL-70 is probably an ideal 1st 4-stroke. It doesn't require a lengthy break-in because it's not a ringed engine design. It's powerful and easy to tune, but tuning a 4-stroke is harder to do by ear when compared to 2-strokes because the lower peak rpms make small changes harder to notice.
If you have an experienced pilot handy with a tachometer who can help you with the high speed needle, you should be up in the air fairly quickly.
Edit: As for 2-strokes, the .61 FX would be a bad choice because of weight. It's 670 grams, about 100 grams heavier than the recommend weight that Nathan King his Sundowner 50 ARF would balance with.
Heavy engines and lead added to balance make for poor flying aircraft.