What size/kind of diode to use for a lawn tractor charging circuit
Other people with B&amp;S engines have had good luck replacing a bad diode with one of the Radio Shack  276-1661 6A 50V rectifiers ($3 for a 4-pack). 

If you're ordering from distributors regularly, a Diodes Inc. 6A1-T (6A 100V) is similar, and about 46 cents each. 

When you locate the diode, if it is of similar size and construction (or a bit smaller) this type should work for you. Google the motor type or tractor type to find the diode location. 

Unless they're physically broken or subjected to huge fault currents, diodes normally fail shorted, so your symptoms are plausible. That kind of thing would tend to destroy the battery over time.