I like that combo... man I really hate to split the case on this thing, but maybe I should while I have it out of the bike.
I prolly should create a thread for the K1 to separate my projects as I await allenp42's coil prescription.
It's all about priorities & goals. I put out a lot of info, most of which I expect will
not be used, so that folks have an idea of what's possible and be able make informed decisions. My suggestion is to begin by setting an overall goal...and your top 3 priorities. That will largely dictate the rest of the project.
On the one hand, I too would hate to completely teardown an engine that is running & shifting like new. On the other, if it's showing signs of fatigue (low compression, reduced power, not shifting as it should, smoking, etc) then it likely needs rebuilding anyway...and building anything with no more than a 52mm bore is not very different from doing a stock rebuild. It's just a few more parts to order.
The CT70, especially K1-later, can be turned into a reasonably competent 50-60mph machine with very few changes to the stock rolling chassis. Oil-tune the fork, swap-in some better shocks (suitable for adult weight) and the rest just needs to be in good operating condition. The stock brakes are fine up to ~55mph, Trailwing tires are J-rated...62mph.