There are crankshaft weights that will fit under many spinners. 1 0r 2 oz at the end of the crankshaft is about the same as 4 oz on the firewall.

SIG also makes weights with a predrilled hole in the center, these can be put between the bolts holding the engine and the motor mount.

If you have the room for the battery on the firewall (either on the engine side or inside the fuselage), epoxy AND SCREW a length of velcro to the firewall, then secure the battery in place. You may need to drill a hole in the firewall for the battery lead (remember to remove the fuel tank BEFORE drilling any holes ) and use a short extension to get to the switch.