Well, you *can* convert "anything" to "anything", if you have enough DIY ability, time, and money, but first we'll need to establish what you really want to do:
Where do you want to ride that? If it's just as stated in the first post, offroad or dirt roads, etc., then you can probably ignore the first two paragraphs below.
If you are trying to be an electric bicycle (the forum you're posting in) on the roads, neither the pit-bike look nor the 45MPH speed are bicycle-like, so there's about zero chance of "stealth". If you want stealth, it's going to have to look like a bicycle and be used as one. You'd have to check your local regulations for what's actually allowed whereever you are, unless you want to lose your ride to LEO action if they do that sort of thing there.
If you're looking for something to ride as a motorcycle (which both the look and the speed suggest) on or offroad, you're going to have to check the local laws for what's allowed to be converted and ridden that way--some places the laws are pretty lax, some much more restrictive.
As for whether you could do what you want (regardless of legality) for the price you're after, that depends on the conditions you want to ride under--terrain, weather, road (or offroad) surfaces, the weight of the system plus rider and anything else being carried, and the speed and range you want out of it under each of those conditions. The motor and trip simulators at ebikes.ca can help you guesstimate how much power it will take, under your specific conditions, which will let you then guesstimate how big a motor, controller, and battery you'll need to do the job you want the system to do for you.
You'll have to sit down and work out exactly what you mean by "rides decent". If you don't quantify that, you can't be sure of getting the right parts to make that happen. Same for any other performance requirements.
You can always just throw more power at it, but that takes more motor, controller, and battery, and those all take more money (and more space and weight).
As for the prebuilt cheap stuff's cost vs abilities, there is a minimum amount of stuff you have to use to accomplish something, and that has a minimum cost. You can use cheaper stuff, but that usually means less capability, less longevity, or less quality, and at some point it's just not enough to accomplish the something.
Sometimes, if you have enough time and patience, you can find used parts that will do what's needed for much less cost than new, as long as you already know what you're looking for (and at). Batteries are one of the hardest parts as they're the most complex and the most expensive for something that can actually perform as needed for as long as needed.
If you go with the PB frame you're linking to, ditching the ICE, you may be able to use one of the bolt-on motor/gearbox units, but you'd have to check that the mounting points and gear output match what you need. Here's a couple of random links from a google search; I don't have any info about either one or their sellers, but there are many similar or identical units available from many places. You'd have to find one with a motor big enough to do the job you need it to do; I think these say 1000w so they're probably not big enough for the speed you're after (definitely not on hills or common offroad conditions):
www.alibaba.com
