That's a tough one, especially if you keep the theme of the board track racer.
I see 2 options, which don't involve the use of the gearbox.
One is to remove the gearbox and use the bottom bracket as a jack shaft.
Use 12t clutch and 44t chainring on the left side.
On the right side, use 24t chainring and 44t rear sprocket-right side.
Gearing would be 8:1.
Good enough without gearbox or torque converter.
Option 2 is how Sportsman Flyer does it:
12t clutch to 24t chainring IDLER SPROCKET.
Then 14t chainring IDLER SPROCKET to 48t rear sprocket-left side.
If using option 2 and idler sprockets, you can install foot pedals.
Gearing would also be 6.86:1.
Good enough without gearbox or torque converter.