Bruce.
It is usually the AUX 2 button on the Cab-1 or Cab-2 remotes that turn the headlight on and off, I never ran a Legacy engine using a Cab-1 but it should turn the headlight on and off. If I were you you I would call Lionel during business hours, talk to a customer service rep and they can walk you through the steps. Tell them you are using a Cab-1 TMCC remote. If they can’t get the headlight on they can give you a RA.
If you have a friend or are close to a train club that uses Legacy see if you can put your engine on their layout and use CAB-2 (if they let you, most clubs are helpful).
Better yet buy a Legacy system, You can usually pick them up for around $350. It is worth the investment.
Good Luck
JohnB