A professional restorer will probably charge $100 per hour for labor. How many hours of YOUR labor will it take to install the existing belly pan? If you use new material you have a good pattern but will have to spend an EXTRA hour or two (assuming electric shears) to cut out the new material.
YOUR CHOICE is determined by answering a few questions:
What is the pain-in-the-neck factor of doing this job - will you curse the day you chose used material when you had to do it all over again?
The existing material does have several mouse highways - through and through corrosion. The rest of the existing material is used and worn at least slightly. So how long will it be before fatigue cracks, more corrosion and plain old use causes the used one to fail? 3 - 5 years? How long can you expect new material to last given the lifespan of the current belly pan?
Your words were that material wouldn't cost that much - which really means it won't get in the way of your ability to finish the project. That's why cost of material didn't enter into what I posted. This is only ONE of a couple hundred such choices you'll have to make - and the questions are always the same!
Happy trails whichever you choose. Have a nice weekend!
Personal opinion: If it were $1100 for a new refrigerator, I'd scrub the used one to a parade rest, re-install it and hope for the best. For less than $400 for the new belly pan... no. 20 years of never having to redo that? Cheap at twice the price. (Changing out a refrigerator versus putting in a new belly pan... reefer is easier!)
Today is a gift, that's why they call it the present.