Bespoke Parts, usually made for Midges

                                     New(ish)                Ok                     Poor     
 Windscreen                   £150                     £50                    £25  
 Dash-top                      £45                       £30                    £15  
 Radiator surround          £70                      £50                    £25  
 Petrol tank                    £100                     £45                     £5  
 MOBC badge (Hex)        £7            
 Midge badge (oval)        £5

​ Generic 
 Headlights (pair)          £200                     £50                   £10
 Narrow Seats (pair)      £80                      £40                   £10

Some addresses that might help with bits and bobs
Small steering wheel (Merlin) Triumph steering Hub  (Merlin)       
Wire wheels 
SC Parts (brace yourself) don't forget the hubs, spinners and nuts
MSW Used to be more expensive but I'm not sure now. Wider range. Check both.
Wheel adaptors (Never tried them myself)
Vehicle Wiring products
CBC (Car builder solutions)

Instruments, I found Mayhill tractors very reasonable for stuff, although their prices don't include VAT or postage (The ignition switch is rubbish though, and the instruments don't all have illumination)
These are just a few, and of course there are many alternatives. More later.
This link will open another web page with a vague idea of what you can expect, when buying or selling a Midge, but remember it is only my opinion, you must decide for yourself.
    Prices, on ebay for example, can be wildly different, and the car may need careful appraisal. 
MOBC   Mk1 and Mk2 Midge Owners' and Builders' Club