You can just flash the MB1 modem, but I wouldn't recommend it (of course, I'm no expert, so take it for what it's worth). It would be much safer to flash the whole MB1 firmware and stay with that (if indeed your reception was better with MB1 than with MF1). You'll have to deal with deferring the MF1 OTA, though, unless you're rooted.
Sent from my rooted, debloated, deodexed Sinclair ZX-80 running CM -0.001 using Tapatalk 4