I'd suggest a B550 motherboard (most likely to work without BIOS update and has PCI-E 4.0 graphics), though other options are: B450, A520, X470, X570 (if the board does not have BIOS flashback, it
must have a compatible BIOS).
You need DDR4 memory (up to 3600, but 3000 or 3200 is fine). Case: ATX if you buy ATX motherboard, or MATX if you buy MATX motherboard.
I'd strongly recommend you follow the troubleshooting path though, because you could buy this stuff and it doesn't fix the problem. Also, DDR4 is really expensive right now, I'd hold onto what you have at first, or buy used.
If you plan on paying new retail prices for AM4/DDR4, you're better off (better value for money) upgrading to AM5 instead of trying to fix this.
My basket at OcUK:
Total: £306.86 (includes delivery: £11.98)