@bitsavers It's been almost 30 years but I used to write a LOT of Z80 machine code and design the hardware around it.
I don't know about a 40-pin DIP, drop-in replacement with an 8-bit ALU, but in later years I designed in quite a few 64180 chips. Those ran much quicker and had a 20 MHz clock speed. But those didn't come in 40-pin DIPs. Lol.