The pony starter sounds like a dying calf in a hailstorm. Partly due to the belt is loose and a bit glazed; I believe the bigger issue is the brushes and commutator needs cleaned and deglazed and the drive gear spiral sleeve and housing needs some light lube. I'm not a hundred percent sure but I'm thinking the starter is 6v. There should be a stamp/mark relaying the voltage rating somewhere on the starter. I heard it was common to use a 12v battery on the 6v starter back in the day. Which heats up the brushes and darkens the commutator (contaminates and oxidizes the copper contacts, CuO) - creating even more heat. Under long periods of contentious use it could damage the stator windings. Think of it as a high horsepower output engine in a race car ... they run great but ya got to keep them tuned or they don't run long. Clean and keep the heat away from brushes and copper contacts on the commutator.
The Generator does not seem to be putting out enough power (key word here is SEEM). The generator may also suffer from the same glazed brushes and commutator but the only way to check the generator's output is to pull out ye Ol' multi-ampmeter and run some checks.
 Fix:
I'll clean up the copper contacts on the commutators of both the starter and the generator with some fine sandpaper as per the O&M manuals (8T, 9K, 7T, form 9008-B & 10492-4). Check brushes and replace if necessary. Lube up the drive gear sleeve in the starter; replace or tighten and deglaze the pony starter and governor belts with some 60 grit sandpaper.
I'll keep updates and pictures as I progress.
Suggestions?
Thanks
 

Thanks for that thought. An often overlooked item for the heart of things. Personal preference here is 12V on the 6V, never burnt one up (yet), but majority of ponies here start fairly easy, so no reason to be cranking away in excess. D4e's former 9U here used to run an 8V golf cart battery and the generator was bumped up to charge it fully. JM

I was told but have no way of checking for sure, Delco black labels are 6V, Delco red labels are 12V on all the Cat stuff of the 1950's. Both on starters and generators.

Careful what you polish the commutator with - can leave carbide imbedded in soft brass.

just for info. i use an optima 6volt battery and it cranks just as good as a 12 volt and the stock 6 volt generator keeps it topped up just fine. if needed extra you can mount two optima 6 volt battery in the same space as one 12 volt.

6 volt is fine if you understand the need for clean connections and good cable , thinking US cars and trucks were 6 volt well into the 50s and still started !!that’s with single grade oils and the starters were little bigger than on our pony motors , just my two cents from the land of Lucas !!