Going from memory the 21 & 28 subframes are identical except for one bracket which I think is for the different rear brake reservoir. The mudguard/under tray does differ very slightly but again should be interchangeable, I recently fitted a 28 under tray to a 21 with no problems.

However a 28 subframe isn't going to help you as those forward mounts on the 28 seat unit attach to cast lugs on the 28 main frame. Plenty of people have fitted 28 seat units to a 21 so someone will be along to give advice but I would suggest fabricating and welding some brackets to your 21 subframe.