As long as you have decent compression in your OS .12, it will push your car with no problem. It doesn't take much to make these little lightweight cars scream.
If your car has a rotary carb linkage, this is the engine I'd recommend. Plenty of power for a lightweight 2WD car like yours, and parts are still available.
http://www.ebay.com/itm/OS-Engines-15CV-X-ABL-Natural-10E-Rotary-OSM11510-/401074138381?hash=item5d61e1ad0d:g:hHsAAOSwWTRWxswz