Here are some photos of an OEM 68 motor and fan, and the PM136.
Fans are pretty much identical in dimensions, just one plastic, one metal. Vanes on the PM136 appear to be a bit more engineered than the standard squirrel cage vanes on the OEM unit. The OEM vanes are slightly hidden behind their flange so the PM136 may look a tad wider but they are pretty much the same. Both about 2.75" tall. PM136 cage is about 1/8" smaller in OD. The OEM motor is about 1" taller than the PM136 ... the PM136 has the motor countersunk into the mounting plate ... which is why the OEM vent tube doesn't fit right.
My 10 amp 0-30 vdc power supply doesn't seem to have the nads to run them full speed so I was testing with a 12v battery. My quickie power meter setup gave inconsistent results. I can say that the OEM unit shows 0.7 ohm across the contacts, and the PM136 shows 0.3 ohms ... which would lead me to believe it is higher wattage. However they are different types of motors, the OEM motor is the free spinning old school DC motor we are all used to, the PM136 has a motor that has that notchy feeling as the poles pass each other ... like I've felt on more modern motors .... I don't know much about motors so maybe someone else knows what this means.
Since differences in the electrical specifications are probably of less interest than how they perform I'm going to try to test them in the housing with a wind speed meter and a manometer to see if I can determine which one "blows" more
. I have an AC/Heat housing with the cores still installed, maybe I can measure which one moves more air.