Given that there is no direct upgrade path to LC+, this may not be practical at any reasonable cost. The only way I can see this happening is either assemble all the parts from Lionel parts, build the wiring harnesses, adapt the tach reader/flywheel to the motor, etc.
The other option is to find a donor locomotive in LC+ and convert it that way by transplanting all the electronics. You still have the issue of adapting the flywheel/encoder to the new motor...
You'll also have to address the tether in either case, though you may be able to adapt the MTH "wireless" tether.
In either case the labor and materials will likely cost more than just finding a suitable LC+ or LC+ 2.0 locomotive and going that way.