14hp Kohler 321 voltage regulator, 1970s

[Fuddy1952]

I have my Grandads Economy tractor he bought new and it's hard to start. A few months ago I bought a new battery (Walmart) 650 CCA, should be more than enough for Kohler 14hp 321 one cylinder.
I charged a few hours, measured 12.6v (Fluke meter), dropped to 8.5v cranking.
I returned battery, Walmart put battery through lengthy test and tested good, actually 717 CCA.
Original regulator/diode, 3 leads, 2 stator pins I get 22.8 vac at 1500 rpm...but 12.6v motor wide open.
So I assume reg/diode is bad.
A few questions:
1) On Ebay sellers have 3 pin regulators for the 321...I assume they'll work ok? They're much smaller than original.
2) I charged battery here on high, it never got over 12.6v engine running. Unplug regulator it drops to 11.8v.
3) there's no parasitic draw...I checked.

I would think having it on high charge rate shouldn't it be 13.5v or so?

 

[Roadworthy]

You do seem to have a charging problem. It should be hanging at about 12.6 volts and when charging it may approach a thirteen volt or higher surface charge. Try charging the battery on a house powered trickle charger. It does sound as though something is wrong with your regulator or rectifier. Newer units may be physically smaller than the original because newer solid state devices are smaller than they used to be.