I need a gas plan engineer to file with Fogaz to install Cirko i

The plan is to document that the installation is done according to code. It usually includes an actual draftsman drawing, which must be done by a qualified engineer, of where the unit(s) will be installed. The costs of the actual installation is then usually bid by the installer. Usually the installer and engineer work together. You can ask an independent engineer about installation costs, but you may find they will give only generic or evasive answers.

Side note: I assume you want this done in Budapest (from your bio since your post did not specify location). In Budapest you will have more luck finding an independent engineer to do the plan, but to that there is a caveat: most things in Hungary are done based on personal relations so finding an engineer to just do the plans in a timely manner may be a challenge.

In other words, to have things go smoothly, you hire an installer who "knows an engineer" to do the plan, or you hire an engineer "who knows an installer", etc. There is a lot of societal pressure to feed these relationships and to avoid going feral and taking on a job without passing on work to "your guy". An American may think "well, if they don't want my money then I will go to someone else", but the fact is the relationship bonds are stronger, and they don't want your money if it means breaking those relationships and there may not be anyone else to go to. And if you do find someone to take on an independent job in isolation, you may be put at the bottom of the list and it may take months to get your plan done. But again, you may have more luck finding an independent engineer in Budapest.

Speaking from experience. Our installer of a new imported product (1) did not know this particular unit needed a plan and thus (2) did not "know a guy" to create the plan. So we had to find an engineer ourselves. After 8 months we are on our third engineer (the last two simply stopped taking our calls) and still no plan. (Which brings in another side note: unless you have enormous patience, avoid being a trail blazer with new technology in Hungary as they may not know what to do with it here relative to the opaque, convoluted, ever changing regulations).