Kathy, welcome!
I don't know what parts fit it. But here's a thought, FWIW: Does that have a compression release? One important difference among lookalike oil filters (from a reputable brand) is whether it contains a drainback valve to hold the cannister full after you shut down. But when starting, if you always spin the engine using the compression release until the oil light goes out, then that drainback valve is irrelevant and any lookalike filter should work fine. Just match the threads and gasket.
Yanmar uses 1064 (small) and 1334 (larger) filters on many models. These are near universal for Honda, and widely used on Subaru, Toyota etc so every store has one. NAPA filters are re-labelled Wix filters (excellent!) so I suggest a NAPA 51334.
Here's a thread where I compared air filters that Yanmar used in the 1970's. Hopefully your 173 uses one of these.
http://www.nettractortalk.com/forums/showthread.php?t=967
Note that the air filter is special and only a Yanmar specialist (or possible a Deere dealer) will have the filters pictured in that thread.
It sounds like you want to put that unit in service right now today. Like Mark said, it might be a good idea to talk to Fredricks and Len and get the correct parts, first.
Here's an old thread that covered near everything. It might answer some of your questions. (Or suggest more questions!)
http://www.nettractortalk.com/forums/showthread.php?t=1269