Re: which motor?
Brand names mean very little. All of the Chinese 2 stroke engines are basically the same. What should be important to you is who you purchase from. Look for a reputable seller; someone who'll be there for you if you have a problem. Look at their warranty and do some reading here about vendors and their reputations.
There are cheap kits available but for the few dollars you might save with a fly-by-night ebay seller you're taking a risk that once you have the kit, if you even get it, that they won't be there when you need help.
You'll get suggestions from members based on their experience with various vendors. Some will be pleased and others will tell you they had problems, either with the quality of their kit or being able to get in touch with the seller after the purchase.
There are some minor differences in the kits/engines but as I said, basically, they're all the same animal.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"