Hello,
I currently have Marathon MOD 5kcr39un6190x pool pump for a 20,000 gallon pool. I’m unsure of the maker of the pump housing. My filter is a Hayward C900 Swimclear plus cartridge pool filter, 90 square foot. The pump is beginning to make noises and I would like to replace it with a variable speed motor. Can I get a variable speed pump to work with the current pump housing or do I have to replace both? Do I need a different impeller for my current set up?
There is a variable speed motor option for your pump. The model number is ECM27SQU, You would not need to change the impeller.
If you post a picture of the complete pump, we may be able to determine the model and recommend the replacement seals.
Thanks for your response. Sorry for my severely delayed response. Here is a picture of my current pump what seals would I need to complete the installation? Thanks again 
The replacement seal kit a.k.a. tune up kit for your pool pump would be the GOKIT54. You have the Sta-Rite Duraglass. Just be sure that if you have a copper cone in the seal plate that you would just need to remove that.
