There are a couple of websites which major on the Powles, however the chances of getting hold of any wiring diagrams are probably extremely remote indeed.
A case of just biting the bullet,getting out the multi meter and laboriously tracking each wire.
Spent hours on last boat merely stripping out all the nightmare birdnest wiring that previous owners had added over the years,
Not a single indentification tag anywhere and some use of what looks suspiciously like electric lawnmower lead.