From the photos, it looks like the arms are made from mild steel rectangular tubing and should be easily repaired by any local welding shop. It would be best if you could remove the arm and take it to a welding shop. If you have a portable welding service come to your house they will still need to bring it outside the house to work on it and they charge quite a bit more per hour for on-site welding than a welding shop. It looks like there is a bolt in the arm that could be loosened and removed to get the pivot end of the arm off. Don't know what the other end looks like, but you should be able to remove the arm without much difficulty. It looks like there is a rubber or plastic bushing next to the cracked area and you won't be able to weld it on the machine without damaging the bushing from the heat. It will need disassembly to do the job right and prevent it from breaking again. A call to the original manufacturer should get you some technical assistance on how to remove the arm.

There are welds present so it seems reasonable it can be welded again.

I should think any experienced welder could put a clamp on it and push it back in to its original location and configuration. Then tack it to stay there while he adds some reinforcement welded in areas where the stress levels are less. Like on the corners.

Bare in mind welding requires alot of heat and heat can cause distortion in machinery. Not knowing any thing about your machine I would also suggest you consider a mechanicle fix much the same only with nuts and bolts in place of welding.

First I think that you need to verify the material make up and take it to any of your local welding shops, they should be able to help you out

From the photographs it is clear that the design of the equipment is not proper as it is not providing enough area to resist the dynamic shear stresses due to cyclic loads.

It is evident from the photographs that this can be repaired by welding because the box section is apparantly weldable. you can get it done by any experienced welder as others have suggested. Make it a through thickness repair as otherwise any stress raiser may initiate another crack because of the cyclic load.

Further, you can improvise on the design of the machine by welding additional flange plates inside the box section to increase the shear area. This will improve its working life.