I've seen a fuel turbine RC jet on YouTube go 451 miles per hour. Technically, that would fulfill the 300 mph start-up requirement for a ramjet engine, so it looks feasible. It would be large, expensive, and dangerous. We are talking fiberglass and reaction engines, not electric motors and foam board. The end result would not be a toy, and would best be done with lots of algorithms and fail safes in a proper UAV setup rather than just a flying model you can take to the park.