Plate amps in commercial subs are often custom sized & labelled by OEM makers- i.e. not built in house - and are often purchased in a contracted bulk lot, and very hard to source late in, or after the end of warranty.
Have you tried contacting Klipsch directly?
A very annoying phenomenon that a buddy of mine who's been a car installer & speaker repair dude for over 30yrs relates is that not only the higher failure rate of plates amps in even prestige commercial brands than with the attendant drivers, but the unavailability of "correct" replacements - period.
Why there's no physical isolation between the amps and working volume of the sub- enclosures in all cases is beyond me.
If repair part is not available, you may may well be faced with finding a comparably powered BASH plate amp and perhaps modifying it or the enclosure's opening to fit.