You can make a very good barrel press for about $20. All you need is a three foot section of black pipe and a set of pipe clamps. Get a small pice of scrap steel and some all thread so you can make a small fixture that will go down inside your receiver to butt up against the back of your riveted in trunnion without getting in the way of your barrel so it can slide back, and you're set. If you LIGHTLY polish the barrel shank with 400 grit wet paper and coat it with Brownell's Barrel Assembly Paste (a fancy name for molybdenum disulfide), it'll slip right in using the pipe clamp. One clamp sit down on the barrel in front of the front sight base and the other end of the clamp sits against the scrap steel and all thread fixture behind the trunnion. All the fixture does is give you a square surface so the pipe clamp can work. Gently screw down the pipe clamp until the barrel moves back (make sure the sights don't cathc on the edges of the trunnion) until you can move a small metal scribe in and out of the barrel pin hole without feeling any resistance between the barrel and the trunnion. Now all you have to do is bang the barrel pin back in and you're set.
I've built three AKs using this technique and it works great. As long as you use the barrel assembly paste, it doesn't take much effort at all to get the barrel back into the trunnion and since the front sight base has two pins holding it in place, you don't need to worry about damaging the sight itself provided you take your time and you're gentle while cranking on the pipe clamp. It really works smooth unless you're some kind of gorilla in a hurry.