Go on give it a go !!
You might get lucky if its just selector related, these you can remove/replace (fiddly) without too much internal dismantling.
If you`re unlucky then the pulling apart is pretty straightforward and, with the aid of utube footage, not too difficult to reassemble.
Things of importance are careful removal of the input shaft, so as not to lose any of the needle rollers - count them and store safely. If you don't get them all during disassembly be wary when you turn the box over as they`re likely to drop out and roll into the darkest corner of your workspace. Same story with the needle rollers, both ends of the lay (cluster gear) shaft, this comes out last so you are at least able to contain them if any drop out; again count them!!
The most irritating part of reassembly is the re insertion of the lay shaft, trying to line up the thrust washers and not disturbing the needle rollers as you thread the shaft through, don't force it, if it wont go smoothly stop and investigate, likely misplacement of thrust washers/needle rollers; same when you come to the input shaft, make sure the needle rollers stay in place.
Actually, if you are only doing the syncro ring, you wont need to extract the cluster gear, only remove the shaft and lower the cluster gear into the bottom of the casing, just be wary though if you have to upend the casing that it doesn't drop out.