Yep Tunnie but the problem is if I don't get lpg installed here at authorized dealer I cannot get a certificate needed for MOT, and I am not able to find any dealer who instlls STAG kits here
Well, you don't need any certificate for an MOT. (EDIT: just noticed you're not in the UK so YMMV

) Your insurance company may or may not ask for one.
If you are getting someone to install it, you need to go with their recommendation, IMHO. Otherwise, any little problem will be blamed on you not following their advice.
Despite the fact that there is very little actual difference between any of the LPG systems, installers seem to have their pet systems that they like and won't touch anything else. (My interpretation of that is that they don't know what they are doing so shy away from anything unfamiliar!).
I think doing an install piecemeal by supplying the system yourself and getting a third party to install it is asking for trouble. Either install it yourself or get someone reputable (that doesn't mean LPGA registered!) to supply and fit.
And yes, the Stag kit is very good, IMHO, and you'll find plenty of people who can offer advice on it here.
Kevin