I offer over 30 years electronic design experience, hardware, firmware, special applications software. In the 1990's I did reverse engineering (redesign for form, fit and functional equal with newer state of the art components) for US Air Force contracts. (I am in US, Texas.) More recently, since 2000, I have done developement of commercial products having embedded microcontrollers. Code development was with ANSI C environment tools. Your product has an embedded device with a Z80 core, a Zilog incarnation of the 8080, so instruction set compatable. Naturally C (ANSI compliant) will always compile in any other development environment. Only hard coded memroy mapped hardware would need to be updated. Assembly will necessarily have to be rewritten. Also do note this applies whether the processor core is 8080 instruction set compliant or not as any newer Intel instruction set device will still have different memory mapping. Practically for such an old design a black box approach, create a form, fit, and functional equivalent with respect to existing I/O I believe is the best. For now my bid is a very rough estimate. I must see much more detail to reach a final bid.