....the engine is tired from it's 50 years of use...
I'm not going to link all the research on Boric Acid basically reconditioning old worn engines here.
It was 'invented' by Argon National Labs if you want to look up all the research.
Basically:
It does NOT do anything to to 'mod' the oil. It just forms an emulsion that is carried around to all the bearing surfaces and pistons, rings and cylinders etc where it reacts with the metal/s to form an extremely slippery ~0.5 micron layer.
So in a bearing or piston you 'lose' 2 microns of wear.
The bottom layer is a ceramic with 85% the hardness of diamond, with the subsequent layers are akin to a deck of micro sized playing cards easily sliding over each other.
You want around 20% by weight of the oil of Boric oxide/acid. (As Boric Oxide absorbs moisture out the air turning it into acid they are basically the same thing.
NB: Boric Acid/Oxide NOT!!! Boracic etc which is another chemical all-together..!!
You want to add this powder to around half a std coffee mug of boiling water.
Pre heat the cup with boiling water; pour that water out and then add more boiling water before stirring in the powder:
The hotter the water; the more of the Boric will dissolve in it, but you wont get it all dissolved and that's fine as long as you get most of the undissolved powder into the oil.
Add that to the engine after making sure the oil level is at or just below full and the engine is hot, then run the engine immediately.
After around 10 minutes you will notice/feel the difference and be amazed by the 'better than newness' and economy etc of the engine.
It will also swing over on the starter much faster/easier, resolving some of your 6V starting issues.
Leave it in for a day or so of use, during which time it will get in under any sludge, loosening it, then change the oil and filter.
You can add a tiny bit of said mixture to the new oil if you like but basically the engine is good for another 100 years before needing another treatment.
NB that most gearboxes rely on the friction of the syncro rings to get your gears spinning at the same speed to avoid grating, so be careful there.
Similarly limited slip differentials become way less limited if you treat them. Great for mpg and all, but...
(A similar thing can be done to bicycle bearings etc guys for that 'role on' and on and on feeling)
If you really still want to electrify it after the above; I would strip out the crankshaft and pistons etc and then fit the motor to the clutch/gearbox inside the empty engine crank housing to preserve its structural integrity and its look.
A water cooled motor then might be best, as has been done and discussed here.
DONT toss said internals: If you ever want to sell it to a collector or someone; the option to take it back to original is very important!
Be sure to preserve said internals, packed away in oil/ oily cloths etc.