I'm not sure what you're talking about? RTGs don't have short decay paths, they have to have long decay paths to last through a mission. P238 is what we use for most probes and has a half life of 88 years. It decays to U234 which has a half life of 200,000 years, short enough to be dangerous but long enough to almost never go away. RTGs tend to produce on the order of 100 watts of electricity from 500 watts of heat. A good nuclear engine will want to use 100+ megawatts when in use.

And more importantly RTGs don't put out nearly enough heat to make a usable nuclear thermal rocket. The important thing is being able to turn them on when you're doing a burn but then turn them off when you're coasting to your destination then turn them on again to stop there. RTGs can't do that.