You probably should consider this a shorter term fix. The high voltage from the magneto will eventually punch through your rubber tubing. Plug wires use a high dielectric constant insulator. Eventually, the long term fix is to replace the coil. Since yours is from the late 1980's it is an electronic ignition and a new coil is about $35, or as little as $20 for an aftermarket. Doesn't make much difference as they are all (OEM and aftermarket) made in China anyway.

You are correct, of course, it’d be best to have a new unit. I had doubts about my fix
even for a short trial but it did work and proved what the problem was. I used clear flexible vinyl tubing inside and black rubber tubing for the outer wrap. Black tape over all. Only ran the motor a couple minutes. Total thickness of the wrap was about a quarter inch on radius.

Plates on one end of the core show signs of separating - additional reason to get a new unit.

My motor was missing the spark plug wire. I just clipped a piece off a used car spark plug wire and glue to hold it in. This on a 60's Tecumseh H60.

"separating" as in alot of rust forming in between layers? or layers
warping? Unless it is rusting alot, that part shouldn't go bad. The layers of metal vs 1
block of metal guide the magnetic field to travel in the desired direction and multiple layers make the field stronger. I don't think a little separating
is a make or break thing (unless u mangle it hitting with a hammer...).