John Gregory wrote:

40' extension ladder... round wooden rungs.
....

How rungs fastened in--captured one/both ends or through hole w/pin?

Either way, probably only practical solution is to drill out present
rung and find/make proper diameter dowel. I'd recommend the make route
w/ clear material as I've never seen OSHA-rated dowel material a la
scaffolding although I'm sure there's a source for the manufacturers.
Guess you could try the/a manufacturer to see of they'd supply a
replacemtn rung, but I'd really doubt if they would.

I've got one of those old suckers too...man, is it heavy to tote
around/set up! But, given price of a new fiberglass replacement and the
infrequency of needing it, I'd look at repair, too, if it were otherwise
still in good shape.