From Partzilla, the 77 KH100 has the same air filter -->> 11013-027.
Using Kawasaki.com for the KH100: 11013-027 = 11013-006 (older part number)

There's a NOS 11013-006 air filter on ebbaayy, but I'd be leery of using an old filter. The foam may disintegrate & the glue may not hold up. I've seen how-tos for making an air filter. The metal end caps might make yours simpler to do?

11013-027
Model (10):
G3SS (1970)
G3SS-A (1971)
G3SS-B (1972)
G4TR (Trail Boss) (1970)
G4TR-A (1971)
G4TR-B (1972)
G4TR-C (1973)
KH100-B7 (1976)
KH100-B8 (1977)
KV100-A7 (1976)

 

I agree you have the metal end caps so with some bulk filter foam you can make your own re-usable air filter. I have done this many times for bikes where the air filter is disintegrated.

 

Well it's been about a year, but I have good news! She finally starting going today! Big thanks to thejunkman for your videos, without them I would've been at a total loss. Now it's time to fine tune the carb a bit and get it on the road!

 

Thanks, that is a flattering compliment, but you did all the work, I just provided a little bit of information and maybe inspiration. The bike sounds good and strong from the little bit I could hear you won't have to do much tuning as it seems pretty spot on right now.