Colm, (I have only just seen your reply - sorry).
I had to fit a 'C' model sub-frame and rear inner wheel-arch. Apart from that, everything fits fine. The bolt holes all match up perfectly.
The front of the inner wheel arch does fit but has to be trimmed down a little on each side to fit the existing brackets.
The wiring is a straight swap (except you end up with 2 spare wires from the old number plate light - not needed on the 'C' model light).
The rear brake reservoir is simply moved onto the rear sub-frame (where it used to be bolted to the main frame. I cut the rubber supply pipe back about 2 inches, but apart from that, it fits fine.
I have kept the 'A' model headlight cowling, as to fit the 'C' model would have involved loads of replacement parts and I think it would not have looked as good as the old one either. It does not touch the new tank on full lock and the standard indicators can also be kept.
The 'C' sub-frame has 4 hooks (I pressume for luggage to be tied on), which I removed with an angle grinder as they have no use for me and look horrible (as well as frequently ripping your clothing I suspect).
The helmet lock can be moved onto the 'C' subframe, but you have to make one of the frame holes a little bigger to fit the lock.
Rear footrests can be fitted onto the 'C' frame (although in a different way) but is an easy thing to do.
The main exhaust is fitted differently, but with the aid of a longer bolt and a large rubber grommet - it now fits better than the original one ever did (with no alteration to the exhaust itself).
I am now in possession of an old 1988 'A' model, that looks exactly like a newer 2001 'C' model - at very little expence & effort.
As the bike (for me) is used purely for fun, it is a better choice for me, but to swap the parts the other way (from 'C' to 'A') would be a little more awkward, as you would need to re-mount the rear brake reservoir (no standard fittings left), replace the inner wheel-arch (it will be to narrow), add a little wiring (number plate light) & move the front indicators somewhere else (you may have to alter the 'C' headlight cowling if it touches the new tank) > as well as replacing all the bodywork. You may also have trouble with the fork stantions touching the tank on full lock.
If anyone is thinking of doing this, I hope this is of help.
I obviously have a load of 'A' model bits spare now (including a sub-frame), which if you ever need - let me know (but with the weight, I would imagine postage to be a bit expensive).
All the best, RAY.