I disagree with Marvin on this one. JB Weld is marvelous stuff, but I don't think it is a reasonable patch material. I think the heat limit is 600 degrees or so. I can't remember the exhaust gas temp, but I think it is at or above that level.
I would start by trying to find a good one out of a salvage yard (may be just this side of impossible).
A nickle sized hole may be reparable if the metal has good thickness to weld or braze to. My suggestion is to take the manifold off, have it sandblasted and/or hot tanked to remove rust and corrosion and then take it to a good welder and see if they can weld a patch over it (actually you probably need to have them look at it first to determine if it can be patched).
Keep in mind that while this is your only hole now, there may be other areas where the metal is weak and thin, so you may have to do this again later.
Keep in mind also that the design of the manifold is to heat the air/fuel mixture to a point higher than optimum for gas so the kerosene would vaporize better, so even though the manifold is a neat thing to have, it isn't the best for pure gas.
Just my thoughts...BB