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There's a band-pass filter at L1, L2, L3, it is used during both transmit and receive.
You require a fairly broad passband,? but I suspect it will work fine across your 500khz.
As designed, it has input and output impedances of 50 ohms (though it likely sees a load of around 200 ohms at Q1).
I suggest you either simulate that filter in LTSpice or find some other software to model such filters,
and find new values for the inductors and capacitors such that it has a passband from 8.222 to 8.746 mhz.
Or you could just make all the capacitors smaller until you bring it to your operating frequency.
For initial testing (not when transmitting into an antenna) you could probably just short across this entire filter
with a wire, but you will now be receiving an image frequency and the transmitter may have extra outputs
at unexpected frequencies..

The low pass filter at L6, L7? allows through our 7.2mhz signal from the transmitter and blocks all harmonics
(at 14.4 mhz, 21.6 mhz, etc).? Again, would be best to obtain correct values for your operating frequency,
though simply decreasing the value of the the capacitors will probably be sufficient.
If this is not done correctly, you may wind up transmitting at frequencies other than what you intended.
This low pass filter is only used when transmitting, not when receiving.

Crystal filter should be fine, leave it at 12mhz.??
If that is not clear, then you need to read up about superhetrodyne receivers before proceeding with
your hacks to the Bitx40, perhaps in one of the excellent amateur radio publications such as
the ARRL Handbook, RSGB Handbook,? or Experimental Methods in RF Design.
There is likely something similar available in most other major languages.

Up to date firmware for the Bitx40 can be found here:
? ??
either "bitx40" (simpler hardware mods)? or "bitx40-raduino-v2" (allows the bfo to be varied)
Adjusting the tuning range in that software should be a matter of changing a few numbers,
folks in this forum could probably help if you get stumped by this.
You will also need to download the Arduino IDE to a host computer, use that to load
your new firmware into the Nano processor embedded in the Bitx40.

Should be a fun and educational project!
Hope that helps.

Jerry, KE7ER