Is it not going into gear because it grinds when you try to get in gear? Or not going into gear because it just doesn't seem to want to 'slide' into gear - but isn't grinding. The shift rail and forks on Allis's weren't exactly their best work. When I went to put a new rail in mine with a deeper detent position (I did it preemptively to prevent any future 3rd gear jumping issues) I was expecting the shift rail to be hardened. I didn't want to just grind a new detent spot in the existing rail, as the detent hole wouldn't be hardened. I accordingly made my own new 'deep-shift' rail from 4140 and sent it for hardening with a batch of other parts we had going out for hardening. But then for giggles I tested the hardness of the original one at work and found it was only about as hard as cold-rolled mild steel (20 HRC if I remember right). My old rail and fork were also wore a little on the sliding surfaces, making for a little slop which I think was making mine hard to shift sometimes. Those parts don't seem to get a good dose of lube up there. After working on it, I now overfill my tranny a little to try and make sure they get some lube splashed on them. Not sure if that helps or not, but it's something I do.

If you do have to go into the top of the tranny, make sure you replace the shift fork for 3rd (or rather, 5th/6th as it's labelled on the 180) with a new one. The new forks you get have a little more meat in the right spots to have a little more positive engagement and help prevent the gear jumping issues.

But first question would be to describe your problem. If not going in because of grinding, it's probably worn springs on the clutch pressure-plate. If it's not grinding but just doesn't seem to want to slide like it used to, it may require some shift rail work. If that's the case, you could perhaps even make it better for a while by lubing up the shift rail and fork with some tacky grease.