GE Dual Wave II - seems low wattage now
 

It is old, but it is built-in with the oven.

Bcause it says dual wave, I am under the impression it has two magnatrons. I think I found the manual here :
Upper oven replacement parts for General Electric JKP67G*02 Electric range | AppliancePartsPros.com

I really am not sure.

1) What is the original watt rating on this?
2) It seems to take longer and longer to heat up a cup of coffee.

This makes me think it is slowly getting worse and worse.
3) Is there anything I can do? maybe clean the fan and what not on the inside?

Thank you in advance
Nick