For those wanting to upgrade the stock BITx40 the favoured option is to use the software developed by Allard PE1NW. This is a well worked over piece of software and is used by 100’s if not 1000’s of BITx40 owners.
Allard’s firmware options can be found here:
You need to choose between his Bitx40 and Bitx40-raduino-v2 branches. Both take some minor hardware mods which Allard has documented extremely well. His version 2 drives the BFO from the si5351 on the Raduino
instead of using the stock analog crystal oscillator for the BFO.