For guys that have done the 125 to 175 conversion what did you do for an air filter? Is the only option a pod filter unless you can find an OE air box for a 175?

What model year are you talking about? 

Just use 125 airbox on every one I can think of. 

DOH! I see the title says 1974.

Someone will correct me if I'm wrong but same exact carb body with different jetting... no new airbox needed.

Looking on the parts fische the 175's use a different air box and the cylinder is large so the carb moves further to the back of the bike. Would be awesome if I could reuse the stock air box, it is what has kept me from making the swap, I have the cylinder and head sitting in the garage

Take a good look at the two cylinders.  The outside is not larger.  Only the bore is.  Carb is mounted in the exact same place. 

I wonder what I have then, cause it is larger overall diameter but the bore size matches a 175. I don't think it is an IT175 the transfer ports look just like the 125 that is on the bike now

Just dug it out and the numbers cast on one side are 44300 and 171cc on the other side, maybe that can help ID it

DT175a/b.

Fins might be bigger for better cooling.

Have you actually tried to install it or are you just guessing?

I have a 74-175 airbox minus the cover I could part with. Came in a box of parts I got with my RT. I saw one on eBay for something like $25 with the cover. 

Unfortunately the magneto 175 air box won't fit a generator/starter 125.  I can't remember positively doing the swap on a 1974 125 specifically, but I have used the 443 cylinder/head on an earlier model and didn't have any airbox issue that I can remember. 

Yamaha even documented the swap on the earlier models and had a parts list to convert a 125 to a 175 or a 175 to a 125.  (Service Bulletin 219).  No airbox change, just the complete top end..  Jetting was even identical between the 125 and 175 on the 69-71.   Different jet specs between the models started with the reed valve, basic carb body was the same size though.

(One thing easy to forget is the piston pin and piston pin bearing...  wider on the 175.  You can use the narrow 125 parts on a 175 as in they will fit, but it's not correct. The earlier "shared" Parts lists are confusing with the two part numbers for pin bearing so I look up a 1977 IT175 to find the 175 bearing number...  for the pin you can use the model prefix to get the correct one.)