You don't need an open source GPS radio for that, just fly a bit slower. The upper limit is plenty fast for weapons, 1900 km/h isn't much of a limitation, neither is 59,000 ft of altitude.

If you can understand the equations and engineering needed to build a cruise missile the GPS equations will not daunt you. Getting the final approach to have a useful Circular Error Probable at anything low enough for an assassination would be more of a technical challenge than the coarse guidance. Unless you had someone shining a designator you’d need real-time machine vision. To say nothing of designing an airframe that can perform precision manoeuvres at speed without breaking up.