Nice looking Joker! It looks like it might be a Ten-Plus ARF (built from an original MK kit). I built 5 of them (3 from the MK kit). The Joker has a pretty large, draggy wing; so more power is better. My first one was built in 1985 and powered with an OS .61RF ABC (before the 'P' version was available). I added an external pump and propped it with an MK 12.5 x 8+1. It flew very well; slowly and gracefully - keeping the pattern at 150 meters was pretty easy. Unfortunately, the ballistic style was still in vogue and I did not get scored well (I frequently heard 'too slow, too close' from the judges). My last one was powered with a Hanno Special which was a nice combination and a lot more power. I tried several props (all APC); the 12x11 works well especially if you want a little extra speed or fly in wind regularly; I liked the 13x9 - a bit less speed, but excellent vertical. The YS longstroke (pre AR version?) has excellent power and should fly the Joker very well. Give us a flight report when you have flown it.
I alway thought the YS .91AC would be a great choice for the Joker, I just never got around to trying it. Guess I'll have to build another one.