Does the capacity of the oil tank need to be precise? I'd like to switch my '77 ironhead from a kicker to an electric start, but I don't want to spend $200 on a tank, so I figure I might be able to make one myself.
No Mike, as long as you have enough oil to get through the engine and back to the tank without sucking air, you will be fine. Harley air cooled use a dry sump system and stores the oil in a tank not the crankcase. Be aware that it may run a bit higher in temp. (the oil) and you should consider changing it more often. 2 quarts will suffice.