If you replace the diodes with higher PIV units you can get lots of power,something like at least 5 kW at 100V.The older alternators usually have better diodes.With a variable frequency 3 phase ac source you could make a dandy synchronous motor out of one.Most car units have at least a 50A capacity and are good for 700W plus at 14 V or around 1200W at 24V and 1,8kW at 36V.So something of an overkill.
As far as using dc motors as generators it should be noted that, depending on the efficiency of the unit the output voltage (for the same current and speed) is only 60-70% of the original motor rating.To get the original voltage& current the speed has to be increased to 1/0,6= 1.6 or 1/0.7= 1.4 the original speed.This is based on a efficiency of 80% as a motor. For instance a permanent magnet dc motor running at 24V,20A,3000rpm with 80% eff. will as a generator put out 19.2V at 3000rpm no load ,and when supplying 20A ,the output would drop to 14.4 V.To supply 24V ,the speed would have to be increased to (24/14.4)x 3000= 5000 rpm and the nl voltage becomes 29V
So either the speed has to be increased and lower output ,or a combination of the two has to be acepted, when using a motor as a generator and a lot obviously depends on it's efficiency or rather the lack of it.