Some assumptions and blanket statements have been made in this thread that are unfortunate. One is that all used engines are unknown and a piece of junk. While there are certainly engines on the used market that should not be flown, the educated buyer can greatly reduce the odds of an unhappy experience. An engine with a well-documented history and recent time in the air should still be airworthy on a fresh airframe. After all, when we buy a used aircraft......we are getting a used engine and many used aircraft are happily flown by new owners.
But not all builders will want to go this route and that is ok.....builder's preference. In the first twenty years of the RV fleet very few new engines were installed in new builds. That has changed as the average cost of a new RV approaches six figures. Yes, there is some risk in installing a used engine, but it can be managed as can all the other risks associated with flying. Putting a 20 thousand dollar engine on a ten thousand dollar airframe is not going to be a viable option for all builders.
Frank, $200 will buy you a brand new gasket set for a Lycoming or Continental.