Hi there,
we had one of these, most of the parts are fairly easy to get hold of.
If you are intent on keeping it original, start hunting for tyres now. The Pirellis originally fitted were special 20" front and 17" rear and difficult to find in useable condition as they haven't been made for years. There are Cheng Shin's available in 19"/17" but don't bother unless really stuck.
It looks like you may need new rims anyway, may be worth considering going for full size 21" / 18". I know these fit on a Mark 2 but you may need to check the swinging arm clearance on Mark 1.
Update your e-mail and I'll send you some pics
Regards
Cubster