We have repaired a lot of multistage, cast iron, centrifugal blowers during the past 25 years. Every one of them could be a story.  It never ceases to amaze me how many different ways people have assembled, or made repairs to, blowers.  Welding cast iron is extremely hard to do successfully. Most simple pin holes, or cracks in the casting, are best patched with RTV silicone.  In fact, assembling blowers using anything but 100% Silicone, is asking for trouble. Other "formulas for caulking" are so varied in texture, heat tolerance, flexibility, etc... that they might work, might not!

For larger holes, steel repair putty's work well.  Cracks in the area of the bore of the cast iron section are of little concern.  However, if chunks are missing, or the integrity of the casting is suspect, replacement is warranted.  Likewise for laby seal castings.  I recently had a laby seal casting with a slight crack along one of the 5 hold down bolt holes.  The shaft seal was still a good fit and there was no leakage from the "crack", so the laby was not replaced. 

Cast iron bearing housings are probably the most important component on a centrifugal blower.  If they don't hold the bearing positioned correctly, a catastrophic failure can occur-wiping out any, or all, impellers, or even the shaft.  Many shops bore and bush the damaged bearing housing, but I don't recommend it.  The repair is only as good as the person doing the repair, and there is no reason to believe the repair would last any longer than a new bearing housing.  Most bearing housings are priced in the $300-$400 range, and any repair could easily surpass that.  Always replace worn, or damaged, bearing housings with new.  Likewise for shaft repairs.  To effectively repair a shaft, the shaft must be completely removed from the blower.  Unless you are really pressed for time(like a 6-12 week wait for new), I would replace a shaft before repairing it. 

Note: there are no bearing shims at the inlet of the blower.  If you find them there, remove them.  Some other key repairs I do not recommend are those to the blower feet, and flange castings.  If the foot, or flange, casting is cracked, I might attempt to weld it to prevent any further damage.  But if any part of the foot surrounding the hole is broken off, the complete head must be replaced!  Same with the flange holes.  There is simply too much vibration, support, pressure, etc.. at stake to chance a repair.  I've stood alongside blowers when they failed and came to a screeching halt.  I know they can spin over onto their side, and they scare me.  When making repairs to blowers, always err on the side of caution.  It may cost a few dollars more to replace, but you could prevent a catastrophic failure!