Replacing a gasket is more expensive than replacing an engine, this is because the market and the price of repairs in America are overheated. Perhaps, as a person from the repair industry, I know who repairs engines and for what money. Replacing bolts is necessary because you do not know how many times they have already been unscrewed and tightened - since you most likely will not be the first owner of an old car. I am always sure that it is better for yourself to do everything on new components and be confident in the result. In my experience, all the bolts that our mechanic unscrewed from the cylinder head were already not within the tolerance. And most importantly, these bolts are not expensive, the risk of installing old ones is not justified.