The moped foam air filter is labeled #7 in the following diagram:
The previous owner had failed to re-install it. Yikes! That might contribute to why I have been having back-firing problems with the moped at top speeds. I realize the 1978 Honda nc 50 foam air filter needs to be soaked and then lightly rung before being place on the moped. The saturation of the oil in the foam air cleaner helps to provide the proper mixture of oil/air.
So, began my search online to find new cheap Honda Express Moped parts that would list the foam air filter part. I shortly discovered the Honda part #:
Found picture of part on http://www.cmsnl.com/:
Was able to find the part on BikeBandit.com part shows up on search result page for Honda Express Element Air Cleaner. It looks like I would pay $11.78 for part with shipping.
I first called local PepBoys Automotive to see if they had some performance foam filter material I could make my own filter from. The guy wasn't very helpful and just said... "No." I also tried calling a local motor bike shop and they told me they could get honda express nc50 foam performance air filter material by the sheet. They didn't have any in stock but they could order it for $10.
I went with Bike Bandit's moped parts and paid the $11.78 for part w/ shipping.