Hi PeterS,
It's a Postie bike!, there are guys in the c90 club who would give their eye teeth for one of those. Probably a better starting point than a regular c90, bigger motor, proper forks, and possibly a 4speed gearbox?.
I've done some checking on the c90 board
http://http://www.c90club.co.uk/index.php?sid=27297ca41e97386835433700e58a4f67
and it seems that they are a derivative of the 6v models, in which case the frame would be identical to mine. There are 'gurus' on this board who can advise on everything appertaining to c90's, I strongly advise you to join.
The frame cut is not essental, by raising the seat an inch or two it could be avoided. In fact, I used the bike for some time before notching the frame, at the time it was the easier option to modify the frame rather than add more padding to the seat.
Conventional panniers (they are actually, easily available tool boxes) were fitted for convenience, that and the fact that in it's previous form it was rather unkindly called 'The Wheelybin'.
I wish you the best of luck with your build, the best advice I can give you at this moment is to do it in stages, in that way you can roadtest each stage to make sure it works for you in terms of comfort and fit. I spent a lot of time re-doing things because I wasn't happy with the result.
It might be better to PM me you want any specific details.
Regards
Ironside.