I believe the retroair kit includes a new pre-86 style lid and metal fan.
As for the SPAL blowers listed they only show one that moves more CFM that the post-85 style blower and it's pulling almost 30 amps to do it. Then you have to cobble up a way to mate it to the existing evap box.
Griffith has the numbers on his website.
Porsche 911 evaporator blower motor tests, Porsche 930 evaporator blower motor tests, Kuehl Wirbelsturm Hurricane Evaporator Blower Motor, 901-624-901-00,911-624-899-00
Converting Griffith's numbers the post-85 blower is pushing just over 500 CFM. Giffith's motor in the pre-86 box is 650 CFM for comparison.
The other option is to adapt a blower/motor from another vehicle. There is an old post here where someone used a Chevy truck fan/motor and claimed an increase. I ended up with a spare evap box and grafted a fan from a 99 ish Civic to the lid. That got me to around 560 CFM. It's not enough of an increase that I'd cut open my only evap box to do it again. -J