Hey Nathan,
I've built 5 Jokers. 4 with retracts and the last (which was an ebay 'salvage' of a partially built kit) with fixed gear. Fixed gear or retracts make no difference in flight performance. Since you're rebuilding and it seems the Joker already had retracts I would keep them. I preferred mechanical retracts in tail draggers (MK gear). My Jokers have had progressively more power. The first had an OS .61RF ABC, but no pump until I retrofitted a Perry pump. The middle 3 had OS .61RF ABC-P engines and the last had a Hanno Special in it (with fixed gear and all monokote it weighed about 7.25 lbs and had great vertical performance). I'd like to build another one with fixed gear and a YS 110 4-stroke in it. Matt flew the Saphir (.60 sized) at the '89 WC with a YS 1.20 in it - very impressive performance.
Good luck and enjoy!!
Will B.