I never really had any shifting problems with mine. It was starting to sound a bit noisy so we pulled it out and took it into the local mechanic. He replaced the cluster gears and some bearings, took it home and put it back in. Drove it into town and noticed an oil puddle under the flywheel cover. Ripped off the cover and discovered that of the 4 bolts holding on the front bearing retainer, 2 were sitting in the flywheel cover and 2 were fingertight. Tightened them up , refilled with oil and went on my way.
6 months later I've just shifted into 3rd and everything locks up sending me into a skid. I quickly discover that any gear but 1st causes the gearbox to be completely locked. Upon disassembly it's discovered that 1st gear is fused to the main shaft. Whether it was to do with the earlier oil loss or not I'm not sure. I had a burnt van with a single rail/9 inch conversion so rather than spending more money I just swapped over the bits I already had. The M21 is still in pieces in a plastic tub at the farm. I got another main shaft for $10 when picking up some Ebay Statesman doors so one day I'll put it all back together.