20/39 was the stock gearing for the Ultra RMK.
Did you order a new chain, too? You likely have a 64W chain and will need a 66W to accommodate those gears.
Personally, I think you're going about this the wrong way... I would never start with changing the gearing because takeoff is sluggish. IMHO, you should be looking at the carbs, clutches, and belt long before you open the chain case.