How do you think the ET90's would sound in my SF Twin?
77 SF Twin Reverb speaker recommendations
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Probably awesome. My SPL graphs of the ET types show a dip at 4.7kHz below the 5.5kHz peak if that's an issue for you. It's generally better for higher gain sounds. Seems like the same speaker cone as the Reapers, but with a wide cloth dust cover that creates that dip and generally evens out the midrange, which makes it a bit less scooped. An even flatter solution might be a G12C or ET90 & G12C/S or ET90 & G12C combo.
AFAIK, the CBS SF Fenders didn't have a blocking distortion filter on the amp section, so they generate more atonal harmonics when driven hard (but break up more gradually). Smoother sounding speakers can cover the harmonic clashing. Either solution should work well (the G12C sounding slightly ~3harder and more chimey). Whether you miss the 4.7kHz is hard to say.
I'd also consider trying a set of Tung-Sol 7581A power tubes at some point with a possible rebiasing. They have a warm flat response and break up smoother than 6L6GC's, like a less bassy KT66. You can usually run two tubes in a push/pull fixed bias amp if you don't need 100W. Usually, you just install the inner pair and run the speakers into half the rated speaker impedance load to keep the tube impedance correct.
Pleased to hear it!