I have a bike that I have bought that someone has seriously messed with the wiring, 
ie ) The headlight is not running off the AC tap of the coil and the batterys not charging amongst other things  ! has anyone got a link or PDF to a close enough diagram or know where  could source a reliable one ?

Welcome from NZ. Any magneto model be it 125 or 175 run the headlight (& tail light here in NZ plus speedo tacho etc of AC from the mag but it should charge something (never enough) back to the battery from one of the other lighting charging coil wires I'm guessing colours now maybe green green/red white, whatever's left over after yellow which will (should) be the highest output. I'm a bit electrically challenged (plus bloody old which doesn't help lol) so what I might do in your situation & if I was desperate, is run the light straight from yellow but tap it off to the rectifier to get some battery charge for brake light & blinkers etc & see what happens. Or better still, wait for some clever cookies to point you too a clear wiring diagram shortly. My vintage iPad is useless but will look later for a diagram. Now me cups teas gone cold. Cheers. Ps, I think I meant then run with lights on all the time or battery would boil straight of the yellow with lights off, or run a single wire AC 6 volt regulator tapped to the yellow to limit voltage lights on or off to save the battery then could run with lights on 'or off'. Ok far from original, so back to the drawing board.

Monoshock or twin shock?  (Guessing twin shock because you didn't add "MX")

Try looking at the 1976 US DT175 or the 1977 DT100 diagrams in the manuals in the Tech Library here...  probably close.
 

Thank you very much for that explanation