Can you mount a scope on a 50 cal Thompson center New England that is not drilled?

It will require that the rifle be drilled and tapped for scope mounts. Some small rifles (usually .22 rimfire) can use a dovetail mount, but those rifles are made with the dovetail grooves, and they will not work with a heavy recoiling caliber. There are a very few scope mounts that use existing rear sight bases to mount the scope, but it will be very far forward on a T/C, and need a long eye relief scope. Your best course would be to have a smith do a professional D&T for your rifle- relatively cheap to do.