20 years old? Seriously doubt it's pure sinewave, those units have much shorter runtime as compared to the cheaper simulated sinewave, and pure sinewave are pretty much a requirement for many active pfc psus, which really weren't around much back then. Most psus were passive pfc group regulated, not apfc dc-dc.
So chances are good that that UPS will match up tech-wise with what's available back then. Meaning if you have a group regulated, older design psu now then it's prolly a good match. If you have a newer dc-dc apfc psu now, chances it's going to be iffy and basically be nothing more than a huge power surge protection as your psu will shut down once the ups switches to battery.