Find a different shop and replace the head gasket on your existing engine. It’s not a big deal to replace a head gasket on a Prius engine, the gas engine is just a regular simple 4 cylinder engine.

You do not want a used Japan import engine. The way the auto market works in Japan is that cars are taxed much more heavily as they age, as a way to strongly incentivize people to replace their cars after only a few model years. One side effect of this is that you end up with a lot of cheap used engines from cars that are more than about 5 years old. A bigger issue here though is that nobody maintains their engines, because they only need to last them a couple years. The Japan used import engines often have never had an oil change! Suffice to say, don’t buy one of if those engines, just get a different shop to change your head gasket. You are right when you say that it’s not a big deal to change a head gasket.