How do I get the Brother DCP -L2540DW to run 64 bit apps on Catalina

You are at the mercy of Brother to provide 64-bit application, and/or driver software. The device is supported by Apple's AirPrint which requires no Brother drivers to print wired/wireless from it. Reset the Print system, and instead of choosing Brother drivers, choose AirPrint instead before re-adding the printer.

Brother may have already released 64-bit scanner drivers for your device, see here, or you may get by with Apple's Image Capture. If this previous sentence is a bust, you might consider the paid, non-subscription, 64-bit VueScan software which supports both flatbed, and document feeder functionality on your scanner without any Brother software or drivers.

Between Apple's AirPrint, and the VueScan software, you would have complete Brother software independence long after Brother stops supporting the device.