Hi there Bertie mate , really glad you made it down , sorry we went on the ride out not long after you arrived and had left when we returned .
Firstly William who's borrowed bike you inspected couldn't answer too many questions as if was my bike he was using . However your question about the black pipe from the back of the head ....it's the vacuum for the Scottoiler .
2ndly the ducks arse ...is a hens tooth

pretty rare. Best search yahoo Japan or either Australia ...where they will spank you ar*e on prices ...or Canada where there seems to be lot left in barns.
As for reg / reg firstly your loom is complete with factory connections see if electrix do one for the 750 Es ...if so its plug n play... You said you wouldn't be touring many miles , so the gremlin shouldn't appear unless you run with th headlamp off.
If the loom has been "modified " any Suzuki reg rec from GS 650 to 1100 cc one will fit .
But as per the 2 at the rally join the 3 yellow direct to alternator wires . Take the black direct to earth of battery and join the red to the red that fed the original reg rec. sorted.