As said practically every body panel is a subtley different shape, as is all the black plastic trim, wheel arches, sills, bumpers, window glass, the rear light apertures are different size and shape.
Some suspension/braking components and front seats can be swapped with minor mods as the basic floorpan chassis/platform is similar.
Try NewMiniStuff for some new upgraded rear lights for 1st gen Mini: NewMINIStuff.com
or for cheaper secondhand try: Mini Spares UK - Mini Spares UK MiniMatt|BMW|MINI|Modifications|Parts|Spares|Servi ce|Tuning|Repairs|Racing BMW MINI Spares | BMW MINI Spares Ltd.
Bodykits, the original BMW option R50/53 MINI Aero-kit is a reasonable price and looks good, if you can still find one........or how about this for more expensive: lohen.co.uk - Track & Fast Road Performance for your New Mini - Online Shop