Servos are always a big question when starting with scale. The servos used in 12th scale are normally mini servos with a high speed motor and torque is not a problem for 12th scale. Below you will find a list of the most used servos in 12th scale.

I have updated the servo from KO-PROPO

Futaba 9602:

S9602 High Speed Mini
 FUTM0107
Volts Torque Speed
4.8V 30.0 oz-in. 0.11 sec/60°
6.0V 37.5 oz-in. 0.09 sec/60°
 
Dimensions Weight
1.4 x 0.6 x 1.2 in. 1.1 oz.

Futaba 9602

Futaba 9650:

S9650 Mini 
 FUTM0260
Volts Torque Speed
4.8V 50.0 oz-in. 0.14 sec/60°
6.0V 62.5 oz-in. 0.11 sec/60°
 
Dimensions Weight
1.4 x 0.6 x 1.1 in.  0.9 oz.

Futaba 9650

1.06 x 0.47 x 1.18 in. 0.80 oz.
Motor Std 3 Pole, Gears P/M, Bearings 2

Airtronics 94761 Digital

Specs
  Torque: 38 oz/in at 6V
  Speed: .11 sec/60° at 6V
  Dimensions: 0.58 x 1.30 x 1.02in
  Weight: 0.9 oz
  Bearing: Dual output
  Motor Type: Coreless
  Gears: Nylon

JR Z3650

JR Z3650:

Specs
  Type: Digital
  Torque: 42 oz/in @ 4.8V, 51 oz/in @ 6V
  Speed: .11 sec/60° @ 4.8V, .09 sec/60° @ 6V
  Dimensions (WxLxH): .58" x 1.30" x 1.02"
  Weight: 1.04 oz.
  Bearing: Single ball bearing
  Motor Type: Coreless Motor
  Gears: Metal Gears

Servo JR Z3550

The KO Propo 949 has been replace by the PDS-951ICS

KO Propo 949
PDS-949 ICS
Speed: @6V 0.09 S @ 60°
Torque: 38.8 oz
Bearing: YES, DBL
Gearing: METAL
Weight: 0.95 oz
Dimensions (WxLxH): 1.42x1.21x0.59 in

PDS-949 ICS

Ko Propo PDS-951ICS 

VOLTAGE: 6V
TORQUE: 108.3oz
SPEED:  0.09 S @ 60°
BEARINGS: YES, DBL
LEAD CONNECTOR:  GOLD PLATED
GEARS: METAL
WEIGHT:  0.95 oz
SIZE:  1.42x1.21x0.59 in

PDS-951 ICS

Now you need to get a Servo saver to protect your server in crashes. the list below are for Kimbrough:

Kimbrough
113 Servo Gear Savers KO, Airt. 23 Spline Drive
114 Servo Gear Savers Futaba 25 Spline Drive
131 Servo Gear Savers Hitec 24 Spline Drive

Source: Futuba, JR, Ko Propo, Rctech