they do have
heat guns at walmart. you're probably gonna need help finding them, as I have no idea as to which department they'd be in. Probably paint or tools.
If you don't want to drop 60 bucks, you can use the oven method, just be aware it'll put alot of stink out when you do it. As the resin heats up, it starts to gas out again, meaning it'll make the apartment smell like resin. You're looking for an overall temp of 200-250 degrees, and you're only gonna need the helmet in the oven for 10-15 minutes. I'd start to pull it out and check every 3-5 minutes, and go from there. You want to keep it in long enough the resin starts to get tacky again, and the overall flexibility of the paper/resin parts to get alot closer to how it was before you resined. just tape it into place before you put it in to get it close, and keep adjusting the tape as you go. It'll get floppy, then once it cools, it'll actually be a little harder than it was when you started.
Another thing I forgot to mention, and which you should be aware of, is that the tape will naturally try to release as it gets hot (the adhesive on the tape itself begins to soften, and will slide off. just make sure that anywhere you put the tape on, give it alot more tape on the helmet to keep it from sliding off.
Also, don't use electrical tape. I know I said earlier that this will work, and it will, but not if you're putting it into the oven. The heat will soften the plastic of the tape, and it won't have any pull on it. So if you're going with the oven, use the fiberglass strapping tape.
The positive side of heating the helmet back up again is that alot of the odors naturally released from the resin will dissipate. Normally, when you glass something, it'll smell like resin for weeks, months, even years. Heating it up will make the fumes go away, as the resin is re-curing. You don't necessarily
have to put it in the oven... leaving it outside on a hot day a couple of times will do the same thing.
IF your forearm part is that warped, I'd suggest starting over. Once the details start to significantly warp, its a huge pain in the ass to get them back to where you want them to be. It'll be less time and effort to remake the part than to try and fix everything with heat, bondo, etc.