First and reverse are on the same shift fork so I would assume something with the shift fork. On my Jinma I had that fork break, it is not a difficult repair and seems to happen fairly often. The top of the gearbox is held on with six bolts, if you remove them you can look in and see what's going on. You don't have to drain the fluid or anything, just take out the bolts and lift off the cover and look in. Replacing the fork is easy, it's held on with a pin that you drive out (I used a hammer and an old drill bit), then slide on the new one and drive the pin back in. Once I had the new part the whole thing took five minutes.
If the fork is broken, don't drive the tractor until you replace it. Make sure you have all of the pieces before putting it back together, a leftover piece can make a mess of the gearbox.
