this is my first build. the engine mount template is for a 2 stroke. i am putting in a saito 100. do i use this same template?

My routine with any model is to first secure the engine to the mount, then position the whole thing on the firewall so the prop hub is centered in the nose opening, then mark the screw or tee-nut holes for the mount on the firewall. So you need to know whether you're going to mount your engine at 90Ëš, 180Ëš, 225Ëš or whatever. I'd be inclined toward a 90Ëš (sidemount), as this is probably the most trouble-free way to go. I would do this before I attach the tail, so I can stand the model on its tail and have a flat horizontal firewall to move the engine around on while I check for centering and mark my holes.

You need to be aware of whether there is any side-thrust offset built into the firewall. If so, then the mount needs to be displaced to the left so the prop hub will be centered side-to-side. You also need to be aware of how far from the firewall the thrust plate is going to be in order to clear the cowl by 1/16" or whatever clearance makes you happy, and this will determine how far out on the engine mount beams you will position the engine.

If there are already holes pre-drilled in the firewall, and they're not positioned where you want them, just plug them with a short length of dowel, glue them in with thin CA, and drill your new holes where they need to go.

If you side-mount the engiine, I would recommend doing a lateral balance on your model before you fly it. You would probably need 1/4 -- 1/2 oz ballast in the left wingtip to achieve good lateral balance. This will help keep you out of a tipstall, improve the plane's stability at low speeds (like on landing).