Mainly for this sort of thing people convert jet fuel starters to turbojet with varying degrees of sucess (JFS100). A small turbojet could be a drone engine like the williams WR24 or CT3201, lots of small starters do not have continuous use oil systems so are no good. I think an air producer driving an air motor would be good as alot of the generator/air producer APU’s are desinged for prolonged use rather than single shots to start and aircraft. People have converted nimbuses to Turbojets too. Oh yes Gas turbines are also very easy to wreck if you dont know what your doing, plenty of videos on youtube to demonstrate that!!