There is a "range repair" some people sell. It replaces the two inductors for the RF system. It involves soldering the old ones off and replacing them.

I haven't researched it much, but you can fry an inductor and it will kill your range. That could be what happened in your case. Search for it and see what you find.

and yeah.... bigger antenna should be unecessary. We know there is a problem to begin with, fix the problem first.