First - front brale - a single disc brake or a drum , should be permitted.
A strict capacity upper limit of 500cc and any single cylinder four-stroke motor of that size.
No 4-valve cylinder heads
Any fuel, including methanol,
No forced induction.
And make it a development class.
In Australia, we had souinds of singles. To win a race you needed a 630cc Japanese motor. But they usually blew up, so the class became out of reach financially.
It is difficult to build a single cylinder motor which is as reliable as a Manx. A Jawa speedway motor almost gets there. BSA Gold Stars and G50s are usually almost as quick.
In the 1960s in Australia the fastest Senior bike was the Henderson Matchless. It had a home-made 4 valve head. It was a couple of MPH faster - not much for all the effort.