I would be surprised if any parts are interchangeable between 4 and 2 brush set-ups but you never know.
Perhaps the easiest thing would be to source a good 4 brush motor and re-furb it or yours because the standard system doesnt struggle therefore something must be amiss.
A really good clean up in the hands of someone who does that sort of thing should sort it all out but after 40 years plus, the supporting ancilliaries may be the issue.
I would replace the battery cables and the lead from the solenoid to starter and from engine earth back to the battery with new stuff.
This works best as it has better density per unit of cross sectional area..
https://www.vehiclewiringproducts.co.uk ... rter-cableThey do black and red and some pre-made to lengt options too with terminals readily done.
New brush kits may be available for what you have now like one of these perhaps.
https://www.electrexworld.co.uk/acatalo ... s_720.htmlThe 4 brush plate is a drop in, dont know about the two brush item.
Solenoid may be gone high resistance so one of these will do as a replacement.
https://www.electrexworld.co.uk/acatalo ... ml#SID=726Its not the only option but i have had one of these for eight years trouble free and the standard one was about £90 or something stupid like that.
Finally the battery CCA may be holding it back.
I assume its in good order but if it isnt then a new one with better Cold Cranking Amps may help.
AL