joining as a CTO but want to keep existing job, expectations

I have been approached by a previous co-worker to become a CTO for a startup that they are creating. Its in the very early stage (company hasn't even been formally formed yet but there is a willing investor already). Its still far from being a "real" company and I'm just using my free time to work out a prototype.

I am currently working as a programmer and have zero experience on being a CTO. The CEO has described what my role would be and I'm confident I can do the job, however, I have a few concerns. We'll be having another meeting this weekend and I'm doing a bit of research.

• Legally speaking, do I have to leave my other job to take on this position? I work for a financial instution in Singapore.
• If I do, is it expected that I will take a pay cut since I'm joining very early on?
• What is the reasonably salary/equity for a CTO joining this early on?

EDIT:
might I also add that this is a software startup. The CTO is just a fancy title, its more like lead developer.