How can I reshape this PCB antenna so that I can still use it with the given circuit?
The matching of an antenna is only one part of the design. This could still be done with a transmission line of a special length. As far as I know there is no generic way to resize your PCB antenna just by a given factor. However there will almost be certainly a different antenna design, which matches your needs and frequency. This might lead to a smaller gain, but doesn't have to. Often there are better ways which can have higher gains, despite being smaller than its predecessor.

Give us some specs. What center frequency, bandwith and antenna gain are you looking for?