The Southern GP is a can motor. On car Tuvesons' site there is a link to a TMCC installed Flyonel SD9. The SD9 is the same body and chassis as a GP. The TMCC board displayed in the photos is the old style TMCC board which was on the large side. The newer mini commander would probably be a much easier fit along with the newer railsounds board if required and there is no programming switch needed. Machining is probably limited to a few drill and taps for screws to anchor the brackets made to support the PCBs. It is better left to the experts to install and that's not free. PCB costs are around $170 for both plus labor. This would allow you to operate the TMCC locos with the legacy controller.
Another option is to purchase a newer Flyerchief GP and install the Southern shell. There will need to be some modifications as the mounting points are probably different. The Flyerchief motor mounting is different and the engine is a better puller than the horizontal can style as in the Southern. Cost would probably be in the ballpark as a TMCC install. This would allow you to operate the loco with the Flyerchief remote or whatever Flyerchief is compatible with.
Another option is to use a Lionel Powermaster in conjunction with the Legacy system. This will enable you to utilize a conventional engine on the same track as a Legacy engine but not at the same time. A switch on the Powermaster allows you to control both via the hand held controller although not at the same time. If the layout is single train only, one powermaster is required. Multiple loops requite multiple Powermasters. Powermasters are available under $100.
