With a non-adjustable timing system, there may not be any timing marks. There will probably be either a TDC or a BDC mark on the flywheel, and there is probably a hole on top of the side case next to the cylinder with a plug in it. Remove the plug and turn the motor until the mark is visible. That's a common design, anyway.
The best way to know for sure is to buy and consult a service manual for the trike. If you can't find one for your SX, the same year Big Red is exactly the same machine with alot more racks and plastic hung off it.