I have a 1991 Formula with the collapsible spare and have removed the original air cartridge. I am looking for a small air compressor that develops enough pressure to fill the spare but small enough to fit behind the passenger seat. I figure some of you may have already done this and asking for opinions here are far better than Amazon reviews. Thanks in advance.

 

I have on of those 12v pump that looks like a cordless drill. Works pretty good, just a little bit on the slow side to inflate my tire. Fits in the rear side storage.

 

Spare probably won't hold air anyway.
And if it did then it might blow.

 

I bought a harbor freight one in a cloth bag for $30. Does the job, but.....NEWSFLASH!!!! You won't be using that spare anyway. Have you tried to air it up and had a good look at it? I'll bet you $100 it's unusable. When I tried mine, one place got a grapefruit sized "tumor" on it, and the rest of it looked far from safe. It's over 33 years old.
Here's your best options: 1) Towing/flatbed coverage on your insurance. (I have Hagertys.)
2) Have a full size rescue spare. I have a spare wheel and tire for my 94. (Both my 91 and 94 Verts use the same size tire.) I keep a 2" spacer in the trunk of the 91, so if the 91 needs the spare it's not a problem.
if I get a flat and I'm not far from home, (I'm generally no more than 42 miles, the distance to my place of business,) my wife knows where the spare is, along with the mini, lightweight aluminum floor jack, and she can bring them to me. Problem solved.

BTW....The 91 came with the inflatable spare, and the 94 came with the compact "donut" spare, and I wouldn't use either one. As a general rule of thumb, any tire should be shitcanned with a date code making it 7 to 10 years old. Worn out or not.Why would spares be any different?

 

I utilize Ryobi One 18volt tools for general home maintenance, thus there is extra charged batteries. I picked up a small air compressor. When I take the Road King out for a cruise, I toss this in the saddle bag. I will also take this along if we are taking a road trip in one of the daily drivers.

RYOBI ONE+ 18V Cordless High Pressure Inflator with Digital Gauge P737D - The Home Depot

 

I may be the ONLY one out there but my collapsible spare looks brand new, inflates perfectly and when testing held air for five days without losing a single PSI. I suspect it may have been replaced at some time. But for now I will use it. My formula has lived it's entire life indoors and has 34k miles. Thanks for the compressor links.

 

Before I used it, I'd be sure and look at the date code. You don't want to be using a tire that might come apart, and could do significant damage to the body.