rear disc

... is no as the drum forms part of the rear sprocket. I'm sure it's possible but IMO not worth it.

To quote Mike Hailwood when he was asked how the rear brake on one of his race bikes performed (by the proud mechanic who's assembled it): "Dunno mate, never used it".

If it's in good condition with decent lining material it should work fine. I think a lot of the replacement brake shoes available today are too hard for use in these brakes. Somewhere like Villiers Services will reline your shoes with more suitable linings.

https://villiersservices.co.uk/index.php?main_page=page&id=9&chapter=4

Ian

Undo the rear dummy axle nut ie the brake plate side nut, apply brake with a heavy foot and carry on applying while retightening the nut.  That will centralise the shoes and improve the brake. If it does not improve then Villers do a good job on relining the shoes and they use the same lining compound RGM use on their best rear brake shoes at 1/2 the price.

Have experienced poor rear brake myself in past, while a new set of shoes will restore performance, so will a little spannering. The above comment about centralising shoes can be effective, particularly if you remove brake plate and file central hole a little bigger so it can move, many are tight on axle and as such restrict effort to centralise shoes. Another possible I have found is to simply swap the shoes over so the lightly worn trailing shoe becomes leading one, can't get cheaper than that.

With regards rear disc I have a mk3 so equipped and it is no better than the drum, if you want better brakes spend your money on the front where it will have greatest effect