Errrr. How do you mean?
By giving them more power? Like the MD-80 JT8Ds had?
Or so the sounds are the same (aliasing the sounds to the MD-80)?

The answer for more powerful engines - here I have given the 727-200 more powerful engines. I've increased the thrust from 15500 to 17400:

[TurbineEngineData]
fuel_flow_gain= 0.003
inlet_area= 18.000
rated_N2_rpm= 29920.000
static_thrust=17400     //15500
afterburner_available= 0
reverser_available= 1
ThrustSpecificFuelConsumption = 1
AfterBurnThrustSpecificFuelConsumption = 0

Answer for aliasing the engine sounds to the MD-80 (a bit of a guess because I don't know which MD-80 you would alias it to):

Create a new sound.cfg with something like this written in it:
[fltsim]
alias=MD-80

Mark