I know this approach may slow your roll a bit, but here is my suggestion...
Many here have vast experience rebuilding the T90. Open it up and show us what you're working with...gear by gear. You're going to have wear on the first/reverse slider and the cluster gear. Unless completely disintegrated or cracked, these can be reused with success, even with a fair amount of teeth wear.
My experience is this; anything with chevrons gets replaced, except maybe the input shaft. This includes the synchros, the complete synchro clutch hub assembly, and second gear. For me, the chevrons are the key to a happy transmission. If chewed up at all, they're out. Additionally, the "small parts" should all be replaced. Roller bearings, thrust washers, etc.
That said, your T90 may have a new second gear. It may have other very serviceable parts I've listed that really don't need to be replaced. So, show us, because one of the three little synchro "dogs/keys" could be broken...and this might be the only part your transmission needs.
