Seat covers and foams do degrade over time. Unfortunately Honda only do whole seats as replacements and these are very expensive £250+, if you can get hold of them.

There are a few specific CRM seat covers around made by third parties but they are becoming rarer. They also tend to be Mk specific, i.e Mk1/2 tend not to fit Mk3/AR but sometimes they will . However seat covers designed for the CR500 (1990-) will fit all CRM Mks.

I am told that some soft furnishing companies will supply foams cut to size and the density required but I have not found/used a company myself. Others have got furnishing suppliers to make and fit seat covers.

Mugen did make a seat for the Mk3/AR which was ~ 1cm taller than the original but these are no longer available.

With a bit of work a CR500 (steel frame) seat can be made to fit Mk1/2 and I suspect Mk3/AR. This gives a higher middle section and a flatter overall profile.

This guy has been used to make seat covers - http://www.motorcycleseatworks.co.uk/