How to repair leaking blower / air exchanger in attic?
Yes, you can fix it.  First, fix the support for the unit.  

The white stuff is a duct sealant.  You can find it at pretty much any big box store in the furnace aisle.  However, rather than messing with that - after you fix the support of the duct work and get it re-aligned, you can seal the gap with aluminum tape.  Not "duct" or "duck" tape - that's not actually meant for ducts.  You want the aluminum foil tape.  Simply apply to completely cover the gap and press down on the tape everywhere with a rubbing motion in order to ensure a good seal.