The plastic 19T pulley for the Xi Sport will be a direct bolt on. The parts tree was pretty low cost, last i checked. Maybe 4 dollars? With a D3.5, you'll probably want an fdr in the 3.5 to 5.0 range. That's going to be very difficult with the stock spur gear. I reccommend getting the team associated 66T spur (again, probably 4 bux). The bigger issue is going to be changing gears without the vertical motor mount. I've done it before, but strongly reccomend against wasting 20 minutes of your life each time you need to move your motor.
Once you change to. 19t pulley, the final drive ratio will b easy to calculate. I'll type th formula below in simple algebra.
FDR = (transmission ratio)(motor gear ratio)
FDR = (38t diff/19t pulley)(66t spur/33t pinion)
FDR = (2.0) (2.0)
FDR = 4.0