What engine will fit in a 1970 fiat 850 spider?

The original US version of the 850 had an 847 cc engine that evolved from the Fiat 600 powerplant except it rotates the wrong way. A 600 engine will bolt in but there are water pump issues and you will have 1 speed forward and 4 in reverse. The clean air act mandated emission controls for all engines over 50 cubic inches displacement. Fiat shrunk the 847 cc engine to 817 cc which is 49.9 cubic inches and under the limit. WHen that exemption was eliminated Fiat expanded the engine to 903 cc and greatly improved its lubrication system. The 903 is the best choice for any 850 bodied sedan, coupe, or spider. Abarth offered a kit that adapted the Fiat 124 engine to the 850 but these kits are very hard to find and tend to be expensive. The kit allowed either the pushrod of with some messing around the 124 twin cam engine to be fitted. The big disadvantage of a larger engine is that the weight bias to the rear gets even worse than with the 850 engine. With primitive swing axles and well over 60% of the vehicle weight on the rear wheels the car can be downright lethal in inexperienced hands. There are several sources like Bayless who can supply all kinds of 850 parts and are wizards at interchangeability. I would be glad to answer questions to the best of my ability at USDSJDM01@AOL.COM. I am not a parts supplier or dealer of any kind-just a Fiat enthusiast.