The OEM air filter for my RT2 is missing and is has a small “cone style” air filter. My experience with this style on the RD400s is not good as they can create pulses. What would you guys recommend for a filter? The bike, I think, is pretty stock tune.

Air boxes turn up on fleabay (there might be some on there now), repro hoses to the carb, I think, are available and the original air filter is still available from Yamaha - it now comes ready oiled.

Alan is spot on... the filters are available but the airboxes are expensive the cage that the filter sits on/around also pop up on ebay.

airbox on eBay

Update, I found a decent air filter housing. Is the stock Yamaha “wire support” and filter a decent way to go?

Yes.

I ended up going with a UNI filter, looks like it includes the inner support. I’m not really sure if it’s an oiled filter or no, didn’t say in the description. This may change jetting if so?

I've found the Uni filter to restrict the airflow, especially on the the '71 RT1B. I use it on my '71 DT1E and '71 RT1B but I cut out the inner layer and washed some of the oil out of it. I don't ride offroad, only on the street after the street sweeper makes 2 passes.

On my RT3s I have the stock Yamaha cage and filter. The filter is still available from Yamaha.

I have several airboxes in excellent condition if anyone needs one.