You will be fine welding on a commercially available steel (say SAE1010) bracket.
It will be equal to or better than anything Suzuki used in the day. Plus it is easy to get, easy to drill and file/cut/machine/weld.
You would want it to be ductile and bend in a crash rather than transmit the loads to your frame in any case. It will support the bike very well if you choose an appropriate thickness - no less than 6mm I would suggest. Probably 8mm if you are tapping holes into the tag to mount the kick stand bracket.
The pivot hole wears out into an oblong hole after a while (where the side stand pivots), so when that happens, you can machine a stainless bush (say PH17-70) to restore the hole like I do on all my worn out kick stand brackets. The staino is hard, and so will resist wear - maybe something to consider.
The bolt will wear out instead so you just replace the bolt from then on.
RM frames of that era are terribly made, awful welding and very badly finished. I doubt Suzuki used anything too exotic in 1981. There may be exceptions but from all I have seen the machine welded joints are ugly and some only partially welded too!