You bet there are leak detectors/monitors. Level indicators retrofitted into your unit would expensive. We HAD glass ports with pretty red lines for level indicators for one of our 2000 ton chillers. It took less than a year for them to start leaking which caused us on the very next annual PM to remove every one of them.
You will probably pay more for the monitor than replacing the unit.
Installing a low pressure switch for the unit is possible. But not something that I would even consider doing. Everything that is installed in the system is a chance for another leak. I know of no dial gauges that are used all of the time. I doubt that they are rated for that application.
Keep it simple, get the unit fixed or replaced