Do you know the history of the transmission and how it was working when it was removed? If you're intent is to lighten gears, etc. that's going to be REAL expensive and unnecessary unless you're trying to build a box for competition.
Rebuilding Alfa boxes is not rocket science and could be carried out by any good gear shop as well as a competent owner/mechanic.
Items required for a minimum rebuild are (about $220):
3 Syncros (put on 1/2/3rd gears; 4th and 5th get minimal wear and should be ok)
1 Syncro Ring Gear (
New Front and Rear Seals
Assumes the following items are good (and usually are):
4th & 5th gear syncros
3rd/4th & Rev/5th gear ring gear sliders
All gears and gear bushings
All shift forks
1. New syncro rings installed on 1st, 2nd, and 3rd gears
2. New ring gear goes between 1st and 2nd
3. Ring gear between Rev and 5th is transferred to between 3rd and 4th and vice versa. (old Rev side faces 3rd gear since Rev is not syncronized and receives almost no wear. 5th gear also received almost no wear)
Here are a couple of web sources for rebuilds, although I can't vouch for their quality or extent of rebuild. Prices are usually around $800 exchange, although I've seen a company selling min rebuild transmissions on Ebay for about $350 exchange. Count on another $100 for crate materials and shipping.
You may contact a local clutch & gear transmission shop as well, if you don't want to do the work yourself. Alfa transmissions are very simple and require no special tools. I'll bet a good transmission man could do the work with the parts and spec sheet for clearance specifications provided by you.
I'm currently in the process of rebuilding my first Alfa transmission. After opening the case and checking clearances and general condition, it looks very good except for the normal wear items listed under the min rebuild parts list. It looks very easy. About the only special tools needed is a dial caliper, feeler gauges and a hydraulic press or large gear puller.
Since it's a spare, it'd probably be worth opening the case yourself and taking a look for obvious wear/damage. You can also check shift fork wear and feel bearings for roughness. At least that way, you'll know what you're up against as far as parts go.