Hey Mike,
Run time is only half of the equation. If the engine sat for a while before you purchased it, it is possible the tires are bad.
Personally speaking, I would change them both as on a steamer they can be a bit of a pain to put on in terms of disassembly but I would rather do it now and enjoy for years vs, having to stop to fix it again if the other tire fails.
I have purchased NOS engines with Zero Run Time and both tires failed within min. of use.
Hope this helps!
Thanks!
P.s. a few extras never hurt anyone