Unfortunetly there aren't many options, and those there are are very expensive unless you do the majority of work yourself. The CTR convertion for instance starts at around £11K from Mike Satur. The KV6 conversion is not straight forward either, as the front bank of cylinder foul the fuel tank and engine bulkhead so there is a large amount of modification required. My main concern with this convertion is the wieght of the engine and its effects on the handling.
I have seen a turbo convertion, but there is not a simple way to do it. The main problems are space and heat disipation. There is nowhere near as much airflow through the F/TF s enigne compartment as with a front engined car, so it doesnt lean itself well to turbos which need to be colled for best performance. Then there is the problem of intercoolers, where to mount one, and the amount of pipework envoloved if you decide to fit it in the front of the car.
The only real foced induction option is the supercharger convertion sold by Turbo Technics. This is around £7K fitted, but is now pretty tried and tested and there are quite a few Elises around with this convertion.
The other option is to stick with a NASP K-series. This is the route I have taken and am now in possession of a 210BHP engine which I intend to fit around September time if I dont sell it before then
There are 1900cc convertions available for the K too, these can be quite reasonable if you do the majority of work yourself. I guess your looking at around £2-2.5K for the parts/block convertion so if you can build the engine yourself and fit it, its quite good VFM.
