I have an average DSA SA58 that I have been wanting to convert over to Israeli style.  I don't plan on going to the extreme of putting an Israeli barrel on though.

I was successful this weekend and obtained a light handguard set.  It went on the rifle but there are some issues.  The handguard screw apparently is different for Israeli HGs.  Also, my SA58 has the short gas tube so the spring/piston is exposed.

How would I remove the gas tube and install a full length one?  

I have a inroad to a stock so that should be straightforward to install.  

Anyone know where to get the gas tube rings that Israeli HGs used?  Full size SA58 gas tube?  What pistol grip did Israeli FALs use?

Go to Gunthings for the parts you need.

Removing the short gas tube depends on whether it's soldered in place. If not, drive out the pin in the gas block and unscrew it. If it is soldered, the pin may also be soldered in place, or it may be missing entirely. Either way, melt the solder, drive out the pin (if present), unscrew gas tube. Clean the gas block threads, screw full-length gas tube in place. You'll also need a gas tube nut (screws into the receiver) to support the rear of the gas tube.