Timko has drop in motor replacements for vertical motored Lionel F3 locos, but not for horizontal motored F3 locos.
Years ago he offered horizontal motor can motor replacements, but the last time I asked a few years ago, he had stopped making them.
It might be easier to put a later frame under the shell, or else find a Williams F3 in Texas Special livery.
Or... put a less worn horizontal motor in it. They are on eBay frequently, as at least one seller breaks down old F3 locos for parts.