An alternator will produce a lower voltage at a given RPM than the voltage required to operate it at the same RPM and power level.
This is because when in motor mode the alternator STILL acts as an alternator but the voltage produced opposes the applied voltage and is called the "back EMF".
The motor current is (Vmotor-Valternator)/Zmotor at the load and RPM concerned.
SO to produce 12VDC you will need more RPM than the motor runs at when powered by 12 VDC and with a power level the same as you wish to achieve.
The above applies to brushed or brushless motors - to DC output motors and to AC output alternators. you COULD try to use a brushed motor but a brushless motor will be generally superior in a number of areas including cost, availability, reliability (no brushes or commutator), choice, ... . AC output from a 3 phase (by far the most common) BLDCM can be converted to DC by using 6 diodes and some DC filtering.
Modern BLDCMs (Brushless DC motors) have a "KV" specification. This is the RPM they run at per volt applied. So eg a 2000 KV motor runs at about 2000 RPM per Volta applied etc.
Assume for the moment that motor and alternator speed are the same at a given voltage.
In your case you want a motor that requires 12V to run at 28000 RPM.
The KV of this motor is thus KV = speed/volt = 28000/12 = 2300.
However, because Valternator will be lower than Vmotor, to get 12V you will need a higher KV motor - say around 2300 x 1.25 ~~= 3000.
Motors are available with KVs from under 100 (rare) to over 8000 (also rare but far from unknown).
Motor wattages vary very widely, but a say 50 to 100W 3000 KV BLDCM will be readily available.
2300 KV, 12V, 80 Watt, about $US11 here
15V (as motor) at 28,000 RPM around 150W+ - here about $US13.
Many more here
Note: As you load the nitro motor down or throttle it back its RPM will decrease, so a lower KV motor will be able to be operated at around the desired output voltage.