I'm looking into doing the same mate, as far as I'm aware it's the head that's pretty much different, and having spoken to a few of the gents that rally them, they are pretty bono proof up to 160bhp.
I'm looking to develop the K4m, maybe strip it right back, check the bores, pistons, crank, and replace most of the seals, new water pump etc.
Then send the head off to be chemically cleaned, port and polished, new valves, car cams and I think as far as I believe I need to replace the hydraulic springs.
I'm not sure If the 133 twinge lump is a straight swap, an also not sure if there are a set of rally cams easily available.
I will also get a 4-2-1 manifold and with a remap it should be around the 140-150 mark with a bit of work to the inlet plenum
