"RPM Range" is kind of an ambiguous term in its two extremes that would be whatever the absolute minimum idle speed is before stalling up to the valves floating on the high end. Perhaps what you are looking for are the factory engine ratings for horsepower? My Haynes manual states the BHP rating is 100 @5500 RPM. Most published information states 86 HP. Unfortunately right around 1973 the method of measuring HP was changed and publication aren't always accurate. Also that 2 liter engine (often referred to as the EAO engine) was made in Germany and used in a lot of British cars so ratings were published for those markets.

 The EAO 2 liter engine is noted for lacking torque down low and it takes modifications to get power out of it in the higher RPM range. In general it would be safe to say the engine was rated between 86-100 HP. The rating seems to be in the 5,500 RPM range. Oddly my Haynes manual has a statement that "Maximum RPM Continuous is 5,850."  That would seem rather extreme and I'd not advise taking a used engine into that territory for any length of time. If your Pinto is an automatic it will shift long before it over revs. If a manual I'd drive it like most manual cars during acceleration, around 3,500-4,000 RPM then shift to the next gear. Cruising (sustained speed) RPM should be in the 2,500-3,000 range.