I have not seen any youtube video's pertaining to the ST's that have not scared me, with the stupid stuff folks are doing, or recommending etc.
That said, Swapping engines out on these bikes is not for the faint of heart.
You will need to invest in some special tools for the swingarm pivot fasteners, etc.
I question what year(s) are the bikes your thinking about?
If they are prior to 07, your probably going to find wire harness issues, crispy sub wire harnesses, damaged SMC's, calipers, clutch slave cylinders, fork seals, bearings, coolant leaks, vacuum leaks, and various other things, that will turn your good intention projects into a major money pit.
And worse yet, your gonna start finding out that parts are going to be obsolete, or the ECU is damaged.
There probably is a valid reason that nobody else has tried to do what your thinking about doing with those parts.
Listen to that little voice in your head, run away.
There are too many still running late model ST's to be had out there, why risk opening up pandora's box with a damaged bike, and a bunch of questionable parts?