Mattjames84 Is mostly carbon. Thus you might want to try using those car engine carbon cleaner and soak it to dessolve the carbon first.
this might help dessolve the soot.
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/362847041626
As car engine have soot built up in the carberator and pistons. And car engine piston is mostly ss as well.
once the soot is dessolved you have a easier work around with whatever remains. Then you can work around with the gunk like polymer (i am going to assume it as plastic, if it is rubber let me know as well).
Plastic have many types, thus you need to use specific solvent to dessolve the plastic. Common types are ABS, LDPE, HDPE, PC, PMP, PP, PS and PTFE. Each have their own specific solvent. I will upload a table of all the common types of solvent for plastics.