Help Choosing a Kit (and a few questions)

not counting the occasional ign failure (carry spares), these are as reliable as the care taken of their build and their use

Honestly, the bicycle motor works one has modifications that basically anybody can do if they just put forth a couple hours of time. I PERSONALLY don't think that worth an extra $100 over a regular kit. If you were to make me choose I'd go with the kings kit, because you can use there generic high compression head then do the modifications that BMW did to it for free. If I were to have a choice of a difference option I'd get a dax motor kit due to his already being upgrade in the bottom end and then modify to your tastes.

I know nothing about this kit, but seems a shame to spend an extra $140 or so for a lot of custom parts that still only does the same 20-40mph that all the other kits do.

But then, I'm a repair guy rather than a hobbyist.

I would not even consider the King option.

They have a 7 day return policy!

If your busy like myself, It will probably take a day or 2 to get started.

Then another day or 2 to finish.

Then another day or 2 of gently breaking the engine in......

By then, you have no recourse should something go wrong (and we all know the quality in these little Chinese gems!).

I also agree, I'd stay clear of Kings due to their short return policy alone... and then the engines BMW sells, they're nice but not worth the extra money when anyone can do these same upgrades at home. I would go for the Dax kit since these have the balanced bottom ends and that's one of the best upgrades you can have for these engines, they're pretty tough to balance at home even tho it's definitely doable at home you'd need to do a complete teardown, ture the crank, then go thru the balancing process, buy and install the better bearings and all, it's a really good deal in it's self due to the amount of labor you wouldn't have to do yourself as well as that most of us don't have the proper tooling to true or balance a crank... that's what makes these engines a great deal.
As far as performance goes, you can get a genuine Fred head for $39 from Fred's site, crmachine.com ,or if you really like the rounded look of the BMW head, you can get his or a generic one from mzmiami on ebay for $40. I have a generic BMW head I got from mzmiami and it's not bad but it does have some sharp edges that'll need to be filed down.
If you want some real performance there's a lot of info on porting the cylinder in the high performance section in here so you can do the work at home in about an hour or so depending on experience and tooling available (dremel, carbide burrs, sanding drums or rolls etc)... or you could buy a ported cylinder from sites like Jake's performance, crmachine.com, arrow racing, and a few others, just avoid ebay sellers you don't know and the larger companies that just import the parts like gasbike, king's, and the like when it comes to the cylinder, there's no such thing as a performance cylinder coming out of China, some may be better than others, but as far as being a true performance cylinder, there are no factory built performance cylinders.

What I would recommend tho is getting a Dax kit and then choosing the head you want and you'll be happy with the performance, lack of vibration, and reliability. If you want a performance engine from the get go, the Dax "Little Evil" engines would be a good engine to start out with...

That's 2 votes for a dax engine! I enjoy my dax bottom end that I built up more so than any other varieties of these engines I have tried.

as a repair guy, I don't see much of them till they break - the trend is that the faster it will go, the more likely it is to be made to go that fast for long periods of time

this often leads to failures

the slower ones tend to be used for necessary commutes, and last longer

I can get about 6 months or so of 10,000 rpm use out of a jug and ring set before it's time to re hone and re ring, but the bottom end just keeps on going. I can re hone a cylinder several times before it totally wears out as long as the plating stays intact, it can be honed. The plating is typically .010" thick and honing takes off maybe .0001"if even that, it's just enough to break the glaze so it'll seat a new set of rings when a ring job feels necessary.

As far as the Dax bottom ends I use, I've never had one fail and these things run very smooth compared to other engines I've used. They have the balanced cranks and better bearings right out of the box so that saves me time and money whenever I build an engine with their bottom ends... and they can withstand everything I throw at them...