Is it possilbe to make up some sort of circuit that would allow you to vary the Ah provided by a battery?
Take one 4.5Ah battery. Fully discharge in accordance with the manufacturers instructions. Charge it to 2Ah. You now have a 2Ah battery. It's discharge characteristics will not be identical to a real 2Ah battery, but for your purposes they are probably close enough.

The easiest way to charge the battery to a known capacity is with a constant current charger. 1A for 2h will charge a battery to nearly 2Ah (due to losses).