Hi Rob, well, I dunno what size the rear belt is, nor am I familiar with the drive system. These are among the reasons that I avoided that model altogether.
You can, in most cases, add an Idler to the bottom of the belt-run on any belt-driven device, so yes, an Idler will solve potential rear belt tensioning problems if they occur.
IF you took out the (usually) flesh colored restrictor, AND it happened to be opened up to the same size as the rest of that orifice, then, it will run better, and typically that will not cause the need for a new jet BUT it may not have been jetted correctly for your altitude and climate, and if that were true then a jet, or more likely a needle adjustment would be in order.
Good luck with your NEr, there is very little info available on this model that I am aware of.
Mike