Most all aftermarket air filters for the 111's will be a big improvement over the stock set up. The stock set up guides warm air into the filter and is restrictive. So looks and price become a big factor in choosing a new air box. At www.IndianAftermarket.com we offer several air filter systems for the 111 bikes. The "Ripper" offers the least restricted air flow. It has a large filter which allows more air but it also flows better as it gets dirty.
I agree with what Marcus has said and his version has been tested by a member from this forum who knows what his talking about and gave it the big thumbs up. I personally run the Indian round Hi Flow filter ,but if our exchange rate was better than it is I would be putting one of these Thunder Works Rippers on my bike without thinking twice.
There was a write up by user Dan_Frisbe about making your own for less than $50. Matter of fact there are quite a few of them (old now). I will see if I can find the links and post them. Once you do start to find them in the archives here it will take you down a rabbit hole but fear not. I followed these instructions and replaced mine with a K&N. Only advice is to be careful about which one you use because some stick out too far and will brush against your inner leg (comfort issue). I went with the K&N RC-0920 from Amazon and I see the price is now $61. But... it will require a tune.
Any change requires a tune change though, that's why I'm sticking with stock. Really want to upgrade the air and exhaust but then you throw the cost of a tuner on it and you have a really big expense on your hands that has to all be done at the same time. Stock isn't slow or un-enjoyable, so I'll just keep riding along I reckon...
I just got the HI PO Stage one and the flash that goes along with it. Big improvement over the stock.
I had a stock 111 and a K&N. The bike just Goes better off the line.
A lot more low speed power. 1st gear is much better I don't have to short shift as much. If you get on it in 2, 3, 4 it has much better power and doesn't need to be short shifted at all. I think the Stage 1 flash should be stock. The Air Cleaner is clearly louder as well. If you really wack the throttle you can hear it. Nice power at 80 too. I typically ride over a Large 1000 ft hill on my commute in the morning and evening and I do it at 75-80 with the cruise on. It is awesome.