I had exactly the problem you describe with my conversion.  On the second
day of a 3000K maiden voyage I suddenly lost power and could only manage
35mph uphill, and 60, after a long acceleration period on the flats.  The
problem was the tube itself collapsing under the extra vacuum of the I4
engine.  I was rather mystified for a fix until I pulled the air filter and
saw all the foil and carboard that had been sucked in.

I replaced the tube with a generic air intake tube purchased at a Canadian
Tire store for about $10.  You'll need to pull the engine vent grille and
remove the "snorkel" so you can clamp on the new tube.