There are several problems with converting a KK 42 to a motorsailer:
Motorsailers typically have much more ballast than the KK 42, typically 25+% of their displacement. The KK 42 has about 5% ballast ratio.
A motorsailer has a SA/Disp ratio of 10 or more. Putting this much sail on a KK 42 adds tremendous stresses to the mast and rigging and the supporting hull structure. You can probably get to a SA/Disp of maybe 5 at best without completely rebuilding the hull to take the loads.
And as noted above, the KK 42 has a shallow keel so it will crab sideways in any significant wind.
So a KK 42 with sails just isn't going to sail very well, maybe giving you 3 kts of speed with 15 kts of wind with no power. That is enough to stabilize and assist with the motor and maybe give you an emergency get home capability.
This article from Good Old Boat Magazine lays out the parameters. It was written by Ted Brewer, a well respected sailboat NA.
https://goodoldboat.com/dissecting-the-motorsailer/
David