Very hard to know where the oil really comes from as not a lot of space to see and a choice of orifices and sources. If the sleeve gear is worn, the chances are so is the mainshaft, which means that a new standard bush or pair of them rather - which is what I think is normally available (Draganfly?) - may not solve the problem on its own. Certainly when I had problems it was necessary to rectify the mainshaft and make a bush (I did mine in one bit) to fit. Re the leak, you could also check that the grub screw - small allen job - that holds I can't remember what (selector rod isn't it?) into the lh side of the casing is present and correct, as its absence would cause a leak. Frankly, if you have to pull the whole thing to bits to sort the sleeve gear (and quite a few bits, clutch etc, just to prove if it even is the culprit), you might as well get an oil seal, bearings and the rest of it, as it's a not-to-be-done-too-often kind of PITA job.
On the other hand, if it's only a little leak, and the thing doesn't self-drain in a big way on the road or on the centre stand . . . I'd probably put it in the non-urgent category and enjoy the decent weather!