I would think you’d need to write a couple of new optimization passes, for example to detect multiplications (in various variants, such as “signed 8 bit to 16 bit, multiplier is 13” or “unsigned 16 bit times 8 bit”) and convert them to LLVM multiplication instructions.

There also is the trick where “BIT” instructions are used to give a function multiple entry points, and that BIT instruction can also be a LDA# (https://retrocomputing.stackexchange.com/a/11132)

I’m not sure that can “simply” be converted to LLVM IR.