Is it rebuildable? Yes. The real question is how much will it cost and is it worth it. The answer to that question comes after disassembly and inspection. Do you have a copy of the factory service manual? If not, I recommend you find one. Ebay is a good source for these. After getting the proper documentation, I would then disassemble the trans and inspect the shafts for any pitting or blueing, the gears for any missing teeth or pitting, and the brass synchro blocking rings for clearance between the ring and gear.
If there is clearance between the brass blocking rings and gears, (maybe someone else here can give a clearance spec) I would not replace the blocking rings. At a minimum I would replace the main roller bearings on the input, output and counter shaft (4 bearings) and the pocket bearing on the input shaft. You will probably also have to replace the plastic scooper that lubricates the pocket bearing. It usually breaks when taken off and it is cheap insurance to replace it.
Disassemble the transmission. Clean all the parts and passages with brake cleaner (or a parts washer if you have it). Check that the synchro hubs for all the gears (3 hubs) all move freely on their shafts. If you previously had a slight crunch going into third gear, you will probably find the 3rd/4th synchro hub is seized on the output shaft and might need to be pressed off. If this is the case, I personally had luck using a little valve lapping compound and a press to lap the hub and shaft so the hub was free to slide back and forth. If the hub won't freely slide, when you reassemble the trans you won't have synch in one of 3rd or 4th. In my case, 3rd wouldn't synch and it ground terribly when trying to engage while rolling.
A hydraulic press is very handy for this job and can be used for other tasks as well. Harbor Freight has a decent one for $200. Or you can take the parts to a machine shop and have them pressed apart and back together. That's up to you.
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