Originally Posted by
Gb200
Or would it be possible to go with some 650b mountain bike rims I already have ... ?
Do you have 650b rims, or an actual wheel set? The Masi website says that your frame has 12 mm thru-axle in the rear. If you have an actual wheel set, check to see whether it's a fit.
I did the same conversion almost three years ago, and it's been one of the most fun conversions I've ever done.