I'm not sure that I understand your question completely, but I'll attempt to respond:
In general, from what I've learned, "push to start" solutions are mainly for EFI engines. I'm not sure if engines that have had EFI added would work, but I'd assume so.
A quick google search yielded the following results:
A push-button ignition system is convenient and offers some security as well. Here is how to install one in an older car.
www.motorbiscuit.com
Ididit has long been the leader in high-quality steering columns and now its new series of id.PUSH Push Button Ignition Systems will replace your standard keyed ignitions.
www.motortrend.com
I believe Flaming River also sells new columns that you can add Push to Start to.
Flaming River - Classic Auto Parts - Vintage Automobile Steering Components - Steering Accessories - Electrical Components
www.flamingriver.com
As far as theft, kill switches can be installed regardless if you have carb or efi.