On the air filter you have, what is it made of? If it is a steel frame covered by the filter element, remove the filter element. It may be some kind of foam or paper. Uni-Filter sells air filter foam in sheets. They have different densities. cut the sheet of foam so it fits around where the original element was. A inch or so of overlap and tie it in place.
This makes a filter element you can take of and clean.
The 74 TX500 was the A model. As in TX500A. 73 was the TX500, In 75 they went to the XS designation. They used the XS to end of production in 78. 75 was a B, 76 was a C, 77 was a D, 78 was an E.
So your Chap with an XS500 was a 75 XS500B or later.
This engine design was a not a predessesor of anything. It was a poor attemp to make an upright twin smoother. Like the TX 750. Both were a failure.
Leo