Fix the 620 because it's the best model A that Deere ever built.
Be aware of the seal location and the drain hole location. Wouldn't be the first time somebody blocked the drain with the seal and made a leak.
1-800-522-7448 for the service manual and the JD web site for the parts manual. Cheaper than tearing the tractor apart again when it still leaks.
If worse comes to worse. Nothing wrong with draining the stinky old GL-1 from the transmission. And filling it with Shell Rotella T1 straight 50 weight. The viscosity numbers are different with gear vs. engine oil. 50 motor=100 gear oil.
