70cc would be a Sachs 4.2 probably, depending on who made the engine it will be a very powerful and dependable engine. Check the end of the crankshaft for a dimple. A dimple means it is a Sachs crank and will be a very good engine. No dimple means it is not so good but may still be useable. Power wise my 4.2 is very comparable to a 3w 75.

Larry