i will have the same problem on my ZVH engine, i have heard to can time it up but working off the top of the piston, if your heads off its easy to find T.D.C. with a DTI gauge. then you can mark the cam pully on the bottom. if the heads on i beleave you have to go thro the spark plug hole and measure this way, its much harder to do and less accurate, but its possible.

im lucky that my heads off so i shall be able to use a DTI gauge on mine, they only thing is, im not sure which piston you set to TDC, i think its piston 1