ORIGINAL: cruzomatic
I just reread your post.
Looks like your engine is scooted all the way back on the mount. Is that the correct distance the manual calls for? If it is and move the engine forward on the mount (away from the firewall) just enought to clear for the bolt I'm sure the extra few millimeters won't be that big a deal and won't have a negative effect. Also, why are you inverting the engine? How about a sideways install? Would that make for an easier throttle linkage installation? Yes, I'd use all four engine mount bolts.
While the model seems well built, the manual is poor to say the least. There is no mention of the distance between the back of the prop plate and the firewall so I have had to work it out the best that I can looking at the cowl. I e-mailed the makers (Green Model USA) on Wednesday and had no reply from them. I had considered a sideways mounting but the cylinder head would stick too far out of the cowl for my liking. The other problem is that the tank and foam around it take up the majority of the area behind the firewall so no matter where the throttle arm is, the tank will always be in the way. Any drilling to the sides of the tank would need to be through the reinforcing triangular stock. I've looked at the dubro four stroke extenders but there would still be the vertical distance issue.