The engines back then used a softer steel screw. Engine sales were so competitive across brands, that reducing production costs were of great essence. It was sufficient to cinch down the head, back, and crankcase noses, etc. It also helped keep cost of the engines down to reasonable. However, modelers don't always select the right sized screw driver to turn the Philips and slot heads, which chew up the screws. Nowadays, of course we simply replace them with socket head screws of metallurgy that is slightly tougher.
Back then though, one could make a trip to the local hobby shop and buy a set of screws to replaced the chewed ones, for a reasonable price.
I've had to drill out the screw heads on a few used engines I bought through the Internet auctions. I have one Fox engine, a .15X, which the stud is buried low enough that I can't get a tool to remove it. I don't have the tools some machinists have to work miracles, and so it remains a parts engine. Sometimes, I have rescued engines that suffered odd fates. One was a K&B .35 Stallion that was water damaged, rusted by masked by exterior bead blasting and misrepresented by the seller as casting occlusions (bubbles in the molten metal while being poured into the mold). Thanks to all Castor fuel, it's interior wear surfaces where it counts suffered very little corrosion damage.
Other times it was a rescue attempt but the patient died on my shop table. Stuff happens sometimes, unfortunately.