Re: New to forum not engines...
Welcome to the forum, Frankie. You've joined a great bunch of people with a world of knowledge and experience with motorized bicycles.
Your question as to "what would be the best engine kit" will be met with opinions based on user experience. I doubt that there is a 'best'. They are all pretty similar. What I will suggest is to deal with a vendor with a good reputation for after sale/customer support. That can prove more valuable than low price.
If you get twenty replies, you'll probably get close to twenty answers as to who sells the 'best' engine kit. It all depends on what luck the person has had with their supplier.
Good luck. Sorry this isn't the answer you were looking for but at least it's factual.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"