Switching a 240 volt 3 wire to a 250 v 2wire
Do not modify your generator!
You need to make a short adapter cable.
Your generator surely has a NEMA 14-20 or NEMA 14-30 socket.  Identify which one it is (post a pic and we can help).
Your generator surely has a NEMA 6-50 plug, but confirm that (ditto).
So you need an adapter cable with the reverse: a NEMA 14-xx plug and a NEMA 6-yy socket.
You can go online and somebody will sell it to you.  Those generally fall into 2 categories:  Bubba in West Virginia builds them to order out of quality parts from the electrical supply house, or the Chinese make them on their fancy injection molder, but they're rubbish.
Or you can step into Bubba's shoes and build your own, with parts from a proper electricacompetent electrical supply house (NOT big-box), and buy three things:

A NEMA 14-xx plug, with xx matching the size of your generator's socket.
2 feet of #12/3 or #10/3 cordage.  This will be black white green.  The size matches the xx size of your plug: 20=#12 30=#10.
An inline NEMA 6-50 socket made for cordage.

Get em together at a real electrical supply, and the competent clerk will make sure the strain reliefs fit the cordage... do not use scrap Romex for the cordage, it won't fit the screws or the strain relief.  It's only a couple bucks to do a proper job of it and have an adapter that will last.