First, regarding a thruster conversion, it would not be my first choice. The shape was a single before and I know there are some people who liked their (convertible) quads run as thrusters, but the feeling is quite different so if you like the board as a quad, I think it is best to keep it a quad.
As for the fcs issue, if you have warranty left, let your shop have a look. If were in any case planning a slot box conversion, you might be able to strike some kind of deal with the shop. Otherwise, assuming the surfinz box is really broken, it can always be replaced with a regular fcs box (some of their models).
Something to try (in the mean time) is also to use a thin shim in the in the box (like a pice of aluminum from a beer can) and also to drip a bit of oil on the screw which will mean less friction and a better feel for the tightening.