Upon closer examination of the picture, I think Frank is correct that the plug is straight up and not angled incorrectly. You have to look real close (I had to use reading glasses!), but the plug does appear straight. If I can see a good pic of the sleeve, I can tell for sure if its a 36X or a 29X, but with the straight plug, I suspect it's a very early 36X in a 35X case.
As for the 35X: Using sport fuel (like 10% nitro, castor lube), it will easily outlast most of us. You only get into issues with the crank when you start re-timing and pumping the nitro to it.
I just purchased 3 very lightly used 35X engines and I will have no qualms whatsoever about putting them on appropriate vintage combat airplanes and running them on 10% w/castor and open venturi's.
As for your 36X:
* IF it has no web in the exhaust, it was manufactured from about 1964 or so up until about 1970. IF it has a web in the exhaust, it was manufactured after 1970, as I recall.
Aside from the "Tigre vs Fox" preferences, a decent Fox 36X will run and run and run if used reasonably (leave it at stock specs) and taken care of (keep castor going through it and don't get crazy with the nitro content).
I have a stock 36X on a Demon, and pulling .018" lines and on 10% nitro, will easily bust 100 MPH no sweat.
Enjoy!