You can fill yer boots with oximeters
here
This one is quite good, but would probably get in the way. Perhaps one could wear it on one's willy?
Might be worth taking the C172 up to Norfolk (CAS base FL200) and see how high it really goes. I wonder what the correct procedure would be to get a non-instrumented engine to the operating ceiling? A cruise climb ought to do it, I suppose, eventually. One cannot use the constant-EGT method without an EGT gauge.