If you want to do this, Buy a crashed or rotten e36 318 IS as a donor(much easier, and cheaper as you know all the components needed work) With regards the LPG, Legally the certificate was issue to the vehicle, not the engine so if you do change the engine then it would need to be set up to run increased/decreased required fuel, However there is no legally requirement for it to be re tested, and to be fair you will only be un doing the low pressure rubber hoses not the high pressure lines, so quite straight forward. Is your current system a single point or multi point? If single point this maybe a good time to change the front end to multi point, it is a HUGE improvement on the single point system. All in all I do have to say……I wouldn’t bother, the performance gains are not really worth the hassle