Guptar, my Christmas gift to you is the following URL where you can read about all the traps and tears to be expereinced by anyone who thinks of adapting an automotive engine for aviation. As a general rule, it is a waste of time and money.
Aircraft Engine Conversions by EPI Inc.
Turbines are expensive because all of their hot section components are made out of the most god-awfully expensive, ornery and just plain difficult materials to cut, shape, form and join on the face of this planet.
Then of course you want some of those components like the cases wafer thin to save weight and trying to form or weld them is an exercise in frustration because of the potential for distortion.
The discs are made of the closest thing we have to unobtanium that is similarly difficult to do anything with, as are the blades.
Then of course you have the bearings, etc. and you want the whole thing to run in a gas stream Two hundred degrees hotter than the melting point of most of the metals at ridiculous RPM's.
The gears on a PT6 are American Gear Manufacturers Association (AGMA) class One, and their are bugger all companies that can make them. Been to P&W Canada, P&W Hartford, GE Lynn (military) and Cincinnatti (civil) you would be amazed at the tears and sweat that goes into any turbine engine.