Although Snapper has sanitized their website and removed most manuals, the parts list is widely available on the web. The manual part number is 7006041, and you can bring up a PDF using a Google search. Since yours is an '88 it would be a Series 5. You can find the appropriate wheel and then Google to find suppliers.

FYI, the part number is interpreted as follows:

21355 = 2 is 21", 35 is 3.5hp, 5 is Series 5, and a P at the end means self propelled.

Snapper never did provide engine parts. You need to go to the engine manufacturer. This vintage of Snapper was fitted with an assortment of Briggs & Stratton, Tecumseh, and Wisconsin-Robin engines. The only 3.5hp were the Briggs 90702 and 92902, and the Tecumseh TVS90 in recoil and electric start. Since you're having trouble finding the air cleaner, I assume you've got the Tecumseh TVS90 with the snorkel hose leading to a tubular element under a chromed cover. The replacement element is 35102 ($15 to $20) and the replacement corrugated hose is 35101 ($8 to $10).

Unfortunately, Tecumseh is out of business and some parts are hard to come by. In this case Ebay and the Web are your friend.

Greatly appreciate the advice and the quick response.

My suggestion is to replace the engine altogether. I found a 5HP vertical shaft engine from a broken power washer and swapped it out with the original. I just paid $50.00 from craigslist for the engine. It has been working good for the last 7 years. I guess it will run for another 10 more years. Still starts on one pull.