I am familiar with an S-port but in this thread https://www.sccoa.com/forums/threads/m90-mpx-s-dyno-verified-400hp.133251/ they talk about an MPX-S and a MPX so i am trying to understand the differenceI think you are referring to the MP S port which is a modified early blower if i recall right and the MPX is a whole new casting. the MPX would be the greater blower.
I did't do any searching so I may be wrong on the names.
I was under the impression there was no 10 rib compatible belt, if this is a direct swap that will not be too long and slip I am going to grab oneD&D Powerdrive 400K10 Amazon about $20.00