1984 B2000 Transmission question.

Hello Mazda people. I just purchased a 1984 Sundowner with 143k original miles for $500. Here's the catch: The transmission doesn't work in third or fourth gear. I drove it home with first, second, and fifth, and am wondering if anyone has had this happen? Haven't had the opportunity to drop the transmission yet. If I do need to replace it, what do you think my best option would be? As far as I know the B2200 5-speed fits perfectly on this 2.0, any other transmissions that will bolt right up?

Thanks in advance for any help, excited to get building this truck!