Correct of course sir, thanks.
Obviously there is more to figuring out any suspension scheme than may seem apparent at first glance. However, that doesn't mean that it has to be hopelessly difficult. Most any sort of linkage scheme will be taxing to properly design from scratch for a couple of reasons. First, calculating the motion and resulting accelerations of the parts would be in order, and second, it is fairly easy to generate very high loads in the various parts making "eagle eye" engineering a bit risky. And then of course, the shock/spring will almost certainly not ideal as it was engineered for a likely much different application. So the value of grafting a complete system from another bike starts looking very attractive. Picking something of similar weight would be good, and replicating the original configuration very precisely is super important. Angling the shock differently to the original application will have just as profound a result as in the simple no-linkage system you have now. If you are designing your own system, keeping it simple will pay off at the end. And you can eagle eye things reasonably safely if you keep a few things in mind. In your case, assuming you will need to keep your current basic shock location and orientation, the lower mount would need to be a bit below a line drawn from swing arm pivot and rear axle. This will make the spring (and dampening) have more mechanical advantage over the swing arm the further it travels, so the suspension will be softer over small bumps, but offer more resistance over bigger ones. Obviously, this makes for a much more complex swing arm fabrication. In selecting an appropriate shock, you can get in the ballpark simply by calculating what the stroke length will need to be, which you can directly measure by moving the suspension through the full travel you want and measuring the distance the lower shock travels along the intended shock orientation. Getting a shock that has this same amount of travel will insure that it will work in the new application presuming the original application weight is similar.
The strength of the swingarm itself is another story altogether - another reason to start with a system off of another bike. It isn't hard to make the arm vastly stronger, but as all things in motorcycle world, the additional framing often conflicts with other things like exhausts, chains, fenders, and of course the suspension which is also taking up a lot of nearby real estate!
I think the simplest "do it yourself" single shock scheme is to copy Yamaha's original monoshock dirt bike design with the triangulated
swing arm and long travel shock assembly mounted high in the frame more horizontally over the carb.
There is another important issue (though less so) which is the very large increase in loading of the swing arm pivot in the frame. Originally, most of the load for the back of the bike was transmitted through the shocks and into the rear subframe and directly to the seat where the biggest single component on the bike is located - the rider. Without the twin shocks, all those loads are transferred into the frame behind the engine. These loads are also often greatly amplified due the leverage incurred by the long swingarm arms working on much shorter distances between the pivot and suspension components. I think often this is does not become a safety issue, but certainly has to have an impact on stiffness and keeping the suspension under good control.