Mike,
Not sure if you have this in new Jersey yet, but what I would use is called paper and a pencil, piece of tape and you're done. Don't know how to download this, but might even be in your drawer with all that stuff, in the kitchen, we all have them.
Here's the hard part, you will need to estimate how much you use this compressor, everyday or week and then multiply this out to when it get's close to 100 hurs. Then get a calendar and see when that is. NOW, this is tricky, you need to write down that date, in the "FUTURE" on a piece of paper and then with this tape, Scotch Tape,maybe, attach it to the compressor.
This may seem like a lot of work, but every once in a while, look at the date on the piece of paper and when you get close, and for stuff like this, 100 hours,120 hours won't make much of a difference.
Let me know if I can help with any other really complicated stuff,
Rich