http://www.electrexworld.co.uk/pdfs/...P-PMG-&-RR.pdf
Make sure to check the stator output when it's hot.
Looks like the new rectum-frier was spliced/soldered rather than using plug connectors - but if plugs, check them for corrosion. Also - get that thing mounted properly so the sub-frame acts as a heat-sink (pref. w/ some dielectric grease on the back for thermal transfer). Even though it's bigger with fins, there's not much airflow there & will run much hotter if the heat has nowhere to 'drain' effectively.
Is it just me, or does the red wire in the pic look like the insulation has gotten hot & discolored?
Use a decent meter & run through the troubleshooting chart - also be aware that when either the stator or regulator goes bad, often the other will follow soon even if it tests fine. At that age, I'd consider replacing both if either tests bad & you have the budget.