What are the laws in your area regarding engine displacement and power output?
Are you concerned about potentially violating any of them?
Pick the most powerful engine you can get that can fit comfortably in the bicycle frame you wish to use.
Which style of transmission do you want to use? Belt, chain, or a combo of both?
Which style of final drive to the rear wheel do you want? Belt, chain or friction?
Are you going to make all the major bits yourself or use a kit/ modified kit?
All these variables should be considered when choosing the engine.
I cannot read the Polish language. Can you provide the dimensions of the engines you are interested in?