I learned to do this - actually both of these circumstances - from a
friend of mine. Yes, you can enlarge the tip opening. If it has
gotten mangled, you can open it out with a hard steel probe,. I have
some antique probes I bought at a flea market. The end tapers from
about one-eighth-inch to a very sharp point in about two-inches.
Burnish the inside well with the probe, and file the tip clean and
square. The normal large size tip is an arbitrary #7. I have made an
equally arbitrary #8 or #9. Beyond that, I just use a rosebud.
The tip is made of copper, or a bronze/brass just off of copper.
Therefore it is forge-able. With very, very light blows, strike the
last one-fourth to one-half inch of the tip held on a small anvil,
or something similar. Rotate the tip to get even coverage all the way
around the tip. In this way, you can reduce the diameter of the
tubing. Clean with a tip-reamer available from any welding supply
store. Again, file the tip clean and square.
After all, the tip is just metal. Restoring the tip, or modifying
the tip, is just moving metal, something we, as a group, do all the
time! I also learned this on a larger scale through forging
iron/steel pipe to different shapes. If you wish, buy a foot or two
of small diameter copper pipe at the hardware store. Practice with
this, and then take the process to the smaller tubing of the torch
tip. You may need to anneal once or twice.