The easiest way to remove the front sight and gas block from your Saiga is to use a hydraulic press. If you don't have one a simple 12 ton press can be purchased at http://www.harborfreight.com
for $99.99. It is also good for removing the barrel from the front trunion and squeezing rivets among other things.
If you want to avoid the hassle, put the pins back in. It is easier to modify the handguard retainer by removing the locking lever. Then you drill, tap and countersink the hole that is left for a socket head cap screw. A 12-24 tap fits the hole in the retainer real well.
After that you take your trusty dremel tool with a fiberglass re-inforced cutoff wheel and slice through the center of the top block and the entire retainer into the cleaning rod hole.(Stop cutting here! Do not cut all the way thru!) When this is done you can carefully spread open the handguard retainer and slip it around the barrel. The final thing is to use your dremel to grind a groove into the barrel in the proper place for the locking screw.
It sounds overly complicated but is really simple to do. You can then install your AK style handguard, slide the retainer into place and secure it with the screw you just installed. You will probably want to refinish the disturbed area with some matching paint. Plasti-kote "Bumper Black" spray paint at an auto parts store. It's a nearly perfect match.
Be careful when grinding into you gun barrel and wear eye protection. Hope this helps.