Not "Cheap" - just specific use.
IF it's on sale at eBay, then by all means...indulge yourself
In this application? Don't overthink it, use a simple wipe, clean up - remove the old paste, and you should be able to retrieve the insulator from that "mess" and just keep it flat and wipe off the old paste. If the part stays on the transistor and looks to be workable/serviceable - the old flat insulator may just need a new coat, repositioned and tightened down with some known - reliable - hardware to keep it in place. Just check to make sure you are not letting the tab short to the back panel (should show more than 1K (1,000 ohms) resistance from back panel to tab)
Some installs may need more attention - as in, wipe down with a zero residue cleaner for prep - just plain alcohol works and this goes back to others that "Sand" down the oxide that anodizing, and expose bare metal - aluminum, steel or any sort of alloy...
Why the concern? Because of potential paths for Galvanic and Electrolytic actions. What was once non-reactive, now is a potential pause for part failure- from the environment it's in. Base metal is now going to start the process of reduction and whether it's (that paste's main ingredient used) zinc oxides or silicone based - the base metal is now exposed to environment.
If you decide to sand down the bare metal to make a better heat transfer - coat well but not so much as to generate paths to short out the leads from dirt, moisture and spare shavings...let alone the dissimilar metals galvanic reactions in the moisture prone environment with accelerants like heat and pressure and power (Current and voltage di-electrical forces) added in.
:+> Andy <+: