Just because it say's 'Gloster Generators' it doesn't mean that the alternator is itself made by Gloster. Many of these generator companies just match bought in alternators to various engines, and sell them on as a complete gen-set, complete with all ancillaries and protection control panels etc. So well worth checking when your back on site...
Stand-by set's run for 24 hrs or more on a single run shouldn't be a problem, unless you mean it's being used as a prime power generator, then you will have problems. If that is the case, i'd be looking for a good few more problems that are going to arise. Far better to advise the customer it's time to purchase a generator manufactured for continuous running!!!