Marks Adaptors no longer do the kit with the R452 transmission because the core transmission was so difficult to get hold of

This is true. It seems like you can basically make your own by swapping 1st and 5th output gears and the coutershaft from the R452 into another R series. Gears are allegedly (I haven't confirmed, shipping to the US is questionable) available at a link pasted in the thread I linked. I have an R452 with Marks adapter, so would be interested in what anyone discovers if they follow up on the parts link for my own future parts needs.