Shimano Altus ST-CT90

Question
I was given a Huffy 21spd. Sledge Hammer bike. Everything functions except the handle bar switches (Shimano Altus ST-CT900). I took one of them apart to look at it and it seems intact but the gear cog doesn't go into the gear right. It seems stuck some. I lubed it and moved it back and forth to loosen it up but that only made it return to the engaged position a little. Seems like the spring hasn't got enough tension to return it to the gear right. Can the Shimano Altus ST-CT90 be replaced by an alternate Shimano? Should I disassemble the shifter and lube, and clean it? I don't have the extra money to take it to a bike shop. I'd like to be able to switch gears. The cables are free enough and the derailleurs seem fine. Right now I have it in second gear. Thanks for any advice.
                             Don

Answer
Don,
Believe it or not, I am answering this from Afghanistan, so it took me a little more time to get to it.  To get to your questions in order:

Can Altus ST-C90 be replaced by an alternate Shimano? Yes, most Shimano rear derailleurs have the same actuation ratio, so they are interchangeable.  What you can't do is put a modern 10 cog shifter with the 7 speed cogset you have.  

Should I disassemble the shifter...?  If it looks rusted and full of grit, maybe.  Most of the time shifting problems do not come from the shifter, so it is usually best to leave it alone, at least in the beginning.  The cheapest, quickest fix is a new set of cable housings.  Even though the cables feel like they move reasonably well, the cable travel through the housing is VERY finicky and the slightest amount of gunk in the housings or rust on the cable will keep it from shifting correctly.  Only if the cables and housings are spotless and new-looking should you try lubing them.  In my experience, lubing the housings is a very short term fix that ends up gunky and leaves me wondering why I didn't just replace the cables and housings.  First things first, try replacing the cables and housings.  

Next check the shifters.  When not connected to the derailleur, do they click to each shift properly?  If not, then you may need to replace them.  Trying to clean them up can be a bit tricky and not worth trying if you can get Altus shifters for $20 online.  

Last is the rear derailleur (although it should be first if it is visibly bent or the pivots are loose).  The rear derailleur can be replaced for $30 if you look around for a deal.  Altus derailleurs are not designed to be user reparable (adjustable - yes, reparable - no).
One thing to note: the Altus system is not Shimano's highest-end system, so it is not intended to last through a lot of use.  If this bike has seen many miles, the parts may simply be worn out and will need to be replaced.

I hope this is helpful for you.  Please ask more questions if you have them...

Cheers,

Graeme