Is it a flathead engine or overhead valve? If its a flathead engine I cannot see any reasin that a headgasket could cause your problem as no oil runs to it.
I wouldrun it for a while and recheck it for leaks after a really thourough wash and degrease, you should be able to see if there is a leak from weeping if you wash it and then run it for a while.
If the inlet manifold gasket is leaking I would replace it, from memory its a rather thick gasket, it would be worthwhile getting a genuine one.
Personally I keep the oil on the full line of any engine at all times, if its bang on the full marker you know if it loses any
Joe.
