There are mags out there,what fits a GMC inline will also fit a 216-261.The mags for the later Chevy inline 6's will also work with a little bit of hand work.The problem is parts,expensive,as it getting them rewound and magnetised if necessary.You can run fixed advance on the street if you get the engine spinning on the starter,then flip on the ignition, but a kickback can break the starter drive.
I run magnetos on my old kick start British bikes,they fire up on one kick if the mag is in good shape.You will have to set the plugs close at maybe .020,or use gold racing plugs like on my bikes.
You might be better off with an old school Mallory distributer,actually the stock single points dist with a recurved advance works quite nicely up to 5000 rpm.
