On the fitment front...
The 124 requires a 69mm tall cylinder. Sound like a big increase? It isn't, stock is 63mm. The engine guard will just fit with a Nice motor...which has a 78mm tall cylinder...15mm taller than stock. Anything built on a 49cc lower end will be an easy fit...induction assembly, brake pedal, footrest, kickstand can all remain unchanged.
You might have to tweak the muffler hanger, slightly, to accommodate the 6mm forward shift; at most, it'd need a little massaging with a round file to open-up the slot. You probably have enough leeway, as-is. With any of the other engine choices, the hanger would have to be modified.
All of the larger (100cc+) engines have different architecture...like comparing a SBC to a BBC...or a Chrysler hemi V8. They're wider, so the footrest asm. becomes an issue, along with the kickstand, brake pedal, engine guard, and induction asm. They're basically all 12v/CDI, so there will be some wiring & electrical mods involved.
The 190cc Daytona lump is an involved topic, all its own. For a machine that's never going to cover many miles...such as a race bike...it tops the cheap horsepower list. For a road bike, that leaves a lot unanswered...like EVERYTHING ELSE. Are there any other details/parameters that would matter to you...or anyone else...whose goal is a well-balanced road cruiser? What will it take to a.) fit the frame? b.) balance the rolling chassis to the power? and what's the longest field service history, i.e. top mileage tota,l and what it took to get there? That's just for starters.
IMO, you've got the right concept, every bike has its place & purpose. Go too far in any direction and there's going to be a different machine that is far better suited. Given enough motivation, time & ability, someone could probably find a way to attach a CT70 frame to an LS3 V8..a gross oversimplification + overstatement that illustrates the point, here.
Kirrbby makes some valid points, things that occurred to me, also. OTOH, so too did yours. What you're really talking about is shelving a 51mm crank, 52mm cylinder & piston and a few small parts. The only things that get upset are the piston rings & cylinder...once they leave the bore, that's it. And it'd be very difficult to remove them whilst keeping everything completely clean. Your existing head, carb & exhaust should handle the horsepower level...and then some. You should see more hp but it's the torque increase that will be bigger. I'd email hondatrailbikes and dratv to find out the differences in combustion chamber configuration, if there are any, compared to a 6V CT70 head. The other key parameter is the total combustion chamber volume of your existing head. Dratv also lists a flat top piston; thus, as long as combustion chamber volume is 14cc, or less, you'd have at least 9:1 static CR if the pop-up slug can't be used. But find out if the recommended piston will work, or could be modified (for valve clearance, if needed) first. There's also the 52x54 possibility, which won't breathe quite as freely, up-top, but sacrifices very little torque. Small bore/long stroke (undersquare) configuration tends to produce higher static CRs. Make no mistake, I'm with Pat on this one...54x54mm is my first choice, given my druthers. Oh, and you'll be using the same 28t cam sprocket...or else(!)...just a longer chain.
The upside of what you're contemplating (reconfiguring your existing motor) is the sheer lack of other changes. Even the outward appearance won't change much. Paint the jug black and most folks will never give it a second glance. You will have to rejet the carb and that's about it. Monitor oil temps. Chances are, they won't be significantly different (once the engine is broken-in). If they are, it'd be easy enough to upsize the oil cooler which, frankly, I don't think will be necessary.
Maybe someday.
