Generally, they will hold up well - it takes some very high temperatures to really affect the temper and strength of the bolts - There are a few caveats:
First, if a bolt is severely corroded (combustion gas, rust, etc.) , stretched (over-torqued) or had been "tight" (much like your broken bolt - but experienced techs have turned removing those jammed up bolts into a Zen practice - Become one with the bolt!) they will be replaced, but that is relatively rare.
Secondly, on *some* engines, the bolts are a "Torque to Yield" or "Torque Angle" - notably some Kohler Command Pro OHV engines, some powersports engines, etc- These bolts are *one time use only* because , as the name implies, you torque the bolts down to their "yield point" where the bolt is stretched beyond any hope of it returning to it's original length upon removal - These are *ALWAYS* replaced, and O.E.M gasket kits typically *include* new head bolts. (and typically the head gasket is MLS - Multi Layer Steel)
Other than that, most head bolts are not so severely torqued that they cannot be re-used, Even after 50 years. However, on old engines, a close inspection of the bolts is always a good idea after cleaning and look for any brittleness (Bright black and shiny, even after cleaning to bare metal) , elongation (typically at the point where threads end and shank begins) and of course, corrosion and damaged threads (damaged threads will affect your ability to achieve proper torque)
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