How can I modify the frequency of a simple 433MHz transmitter module?
The frequency-selective component is the SAW, so you have to replace that.
Coil L1 isn't critical (it just decouples the power supply), but coil L2, and the T1 transistor, and C2, are matched to the antenna; you'll
want to scale L2 and C2 up slightly from the 433 MHz values, and
change the antenna length (lengthen it) for the lower frequency.
As a cut-and-try approach, consider boosting C2 by a factor of
(433/304)^2 ~= 2, and just leaving L2 as-is.  The antenna length will be about 25 cm.

More troublesome, the C1 and C3 components are to match the SAW
impedance; the SAW you'll be installing may work without changing
those, but only full specifications will tell.   It needs to
drive the base impedance of T1, and not support spurious frequencies.