It would be easier to make suggestion if you had an RPM range but, since
the engine is under 400" and guessing peak power at 7300.........

1.875" x 25.5" w/3.5" x 15" collector

OR

1.875" x 13" stepped to 2.0" x 13" w/3.5" x 15" collector

I can give you specs for 3-step too.