Can the small voltage output from a dc hobby motor (used as a generator) be amplified to light an led or flashlight bulb?
Be aware (you may be) that you can step up one or other of voltage OR current but you cannot "step up" energy or power. If the input power is not enough to supply the load requirement, adjusting voltage will not help. 

That said, a small motor will usually provide enough power and you usually need to change the voltage.   

Small motors that run on DC and that have "brushes" make DC output when used as a generator. 

Motors without brushes such as 5V or 12V fan motors, make AC voltage when used as an alternator BUT this is usually blocked by internal electronics which must be bypassed for them to be used as an alternator. 

A motor with DC output can use a "Joule thief" circuit as mentioned by Jim Dearden - but these are usually super horrible circuits with low efficiency. Often slightly better is to use the electronics from a cheap single cell "lawn light" which droves an LED from one NiCd or NimH cell. (A very few use LiFePO4 cells which are less suitable in this application). There are other alternatives but comment on whether this meets your needs.