It's jus' my personal opinion ofc but...
The only reason I can fathom as to why the Predators & the 212s might be appealing to anyone is price alone... IMHO, they're really ill suited for motorizing a bicycle & a kit isn't gonna change that. While seemingly a "great deal" at first - in the long run I think it's a case of false economy.
While a "kit" option may make it easier for the novice to cob something together - it will still result in something that can hardly be called a bicycle in anything other than name, these oversized, overweight four stroke utility engines really don't have any advantages that I can see other than the cheap novelty factor, building it because you can... while that's a valid reason in it's own right, it's the only valid reason I can see.
There's no aesthetic appeal with such an engine, there's little to be done to make it look like anything other than it is, a cheap water pump motor stuffed in a bicycle frame. Granted, looks aren't particularly important - but at a minimum of 11 & 1/2" wide, 27lbs yet still only 3.5HP even though it's twice the legal displacement almost everywhere that motorized bikes are allowed - it doesn't even make much sense from a preformance standpoint & it sure as heck isn't going to be a practical pedaler anymore.
Add in the costs of a mounting kit, clutching mechanism & jackshaft assembly and all the assorted peripherals (fuel tank, sprockets etc) and it starts making less sense financially too, you're starting to get into the kind of money and time it takes to just wake up any of the in-frame two strokes, to get 'em close to the same power & far more reliable yet still retaining a comparatively lightweight machine that's easy to pedal, one that can still be called a motorized bicycle and not a DIY motorcycle with a substandard frame & running gear... even if you choose to go with a 66cc, there's a world of difference between 16cc over and double the legal size - seriously, just peeling stickers off/adding your own isn't going to fool anyone with a monster like the HF.
Even if you don't like the Chinese in-frames due to the lack of quality control & reliability issues - for the amount of time, money & effort the Predator will end up costing you, you could spend the effort it takes to utilize any number of the other engines available, with equal or even greater preformance - even if you choose to go with a 50cc. Not only are there a plethora of lightweight, preformance two strokes that while not "kits" only require the same amount of fabrication & ingenuity as abomineering a utility motor in a bicycle frame (& remember, less weight = better preformance), there's other four strokes of equal & even better quality than the HF - ones that have nearly the same HP, equally wide yet come with their own intrinsic gearbox, a proper transmission of your choice (yet still the same weight as just the HF alone), 3 or 4 speed, auto clutched or full automatic w/o all the fuss and muss of some overpriced aftermarket cob job.
While currently those four strokes may only be available domestically as horizontals, given you'll be getting almost the same HP with 1/2 the displacement even stock (and unlike the HF there's a vast selection of aftermarket preformance parts) I think it's worth the trouble to find or modify a bicycle to fit - it'd cost about the same, even more reliable yet w/far better potential for preformance mods & best yet you could ride it on the street w/o fear of impoundment.
Please don't get me wrong, I'm not one to dismiss the joy of tinkering & experimentation w/cries of "get a motorcycle" - as far as I'm concerned it's your time & money, build what you will & ride it like the devil... it's your life & your responsibility. If you know & accept the potential consequences for what they are, who am I to say what you should or shouldn't do? I've nothing but respect for the DIYer & the ingenuity such requires...
...I just can't see the HF engines as anything other than a misguided attempt to save a few bucks building something, seeing just the 99$ price tag & overlooking near everything else *shrug* I think it's a bit of a shame actually, so much more could be achieved with a little bit more research & determination.