Re: nissan 18hp four stroke timing gear marks
The timing marks are on both the cam (driven) gear, and crank (drive) gear. They should align when the belt is installed.
This is an interference-fit motor, so you do not want to guess, as that could result in a bent valve or worse.
A Factory service manual is available from any Nissan/Tohatsu dealer, and well worth having for work like this.