Thanks for chiming in FbB. sa2405, modern stems are usually described with all of the following:
1) fork steer tube diameter
2) length
3) rise, in degrees from the steering axis
4) handle bar diameter
Obviously, length and rise are all you have to work with here. Depending on the rise, you could get the bars closer to you and higher, further away and the same height, or further away and lower with a 120 mm stem. I would go to a bike shop and see if they have something cheap or used to try out, swap, borrow, or purchase outright. I don't think 20 mm will effect the steering too much, but you might be surprised and are smart to ask that question. Back in the pre mountain bike era any thing over 120 mm was somewhat rare, and considered a detriment to handling.