Does this frame have curved tubes? If so, this might get a bit involved but, the best projects do that somehow...

Cut off front triangle as suggested above, source the right dimensioned 4130 tubing - you could even go thicker walls to give extra stiffness or whatever you're looking for. Build it from there. There are several sources for 4130 steel in many different dimensions. Error on the side of thicker walls so you have material to machine away to fit headsets and such.

Maybe just build a new frame with design based on the Workman but with 4130 (which the Workman might not be) and you'll probably end up with better results.

fun project!