That's basically the problem. You can build a pretty decent reciprocating engine with some steel and a good machine tool, but turbines are high-precision equipment. They often run hot and need exotic materials. Companies can and have cranked out new aviation recips using CAD-CAM resources (The Rotec and the Sadler radials come to mind) but AFAIK no one has done anything similar with a turbine.
Coupled with their higher fuel consumption and the fact that they're more efficient up high, it's hard to come up with a design that'll be suitable for the run-of-the-mill GA airplane.
Guy came to our EAA meeting once with a Solar T62 APU conversion. Pretty neat package, and I had fantasies about putting one in my Fly Baby. The ONLY thing that made them anything near economically feasible was the availability of used T62s on the surplus market.
Ron Wanttaja