An update on the thermal plates Aavid Thermalloy Polyimide, there is no indicator of modified parts in the material, nor incompatibility with frequency switching devices, all tho I am still not 100% on them for that use.
In amplification use for switching supplies, at times the frequency of the switching can respond to certain materials used in thermal pads, all be this is quite low to not even be noticeable in regards to amplification uses. It's just that some insulating thermal plates can cause slight issues with actually affecting switching speeds that far exceed you amplifiers capability. Therefor that range has been excluded from my reasoning on the Thermalloy part.
As for general use as a thermal pad and electrical insulator in this particular amplifier project, I see no problem, just as long as it fits via the size spec for that part from Mouser, with the part having any issues in this application. Main concept it is a bit overkill tbh. Tho with the tensile strength of 25,000 PSI, as indicated by Mouser's listing of specs. And a temperature range of -260 C (-436 deg F) Cold range to 240 C (464 deg F) Hot ranges. And a thickness of 0.05 mm (0.002 inch). And if by any means the color Amber is not an issue to you, then the part fits the part, so to speak. All be it just a tad bit smaller than those original mica plates.
Also my spelling of Capton, it's actually spelled Kapton, (bad habit on my part) for the material also present in these Thermalloy pads.
They show to be usable in you application based on specs mentioned from Mouser and the Aavid site.
These pads in the datasheet are the 43-77-20G, that places them at the For TO-220, TO-126, Case 77, Case 199, Case 90, TO-218, TO-3P
.
The TO-3P are the closest match up to your driving transistors, just a reference to the housing, this data sheet is a reference only, and is very not likely to be a correct match to yours. Just the package housing parameter in the Package Dimensions section.
https://www.onsemi.com/pub_link/Collateral/ENA0188-D.PDF