It is what you prefer, some guys buy an all original just to resto mod it, and the next guy pieces one back together, and other wait for the cream puff to show up.
It really depends on what you want i.e. a project to keep you busy, turnkey jump on and ride, how fast do you need to go or wnt to go? The biggest setback for some is the budget, what can you afford to spend?
IMO, to find a HK0 motor within + or - 500 of the frame number is going take some searching and patience, then once said motor is found how much will it take to properly rebuild? l'm not too up to date on rebuild prices lately, but I short while ago, I read around $1200 for a total motor restore done right and back to oem. Now don't get me wrong you could buy a complete original bike running and driving and still stick some coin into "freshening" it up.
Long story short, set an end goal of what you want, (complete? project? resto mod, etc) what are you going to do with the bike? putts around the neighbor hood? or camp ground? or tear ass across the city street? (the need for speed wont happen from an oem motor) then incorporate that into your budget. that is why post toilet paper pandemic the market is flooded with oem honda ct70 frames with aftermarket engines. fairly cheap to sand blast, prime and paint throw in a $250 125 lifan and boom you have a cheap motor and more speed.