Those are a twist on type and you push it on then twist, they stay on.. That particular plane is a fully cowled pattern ship and you could get to the plug cap with a needle nose, maybe? Never had one come loose.
Gene, my LHS has a big book that has all there products shown and they order them for me. The original owner would always double my order thinking he would have them on hand but they always sold in a week. Not sure if the new owner stocks them or not. I have to go there in the next week and I can find out. If you want any of this stuff write me and let me know. If I'm out I can stop by Radio Shack and get the makens too. However, the LHS is now under new ownership and the guy doesn't like to be bothered. I'm out of the caps, when I built or set up a plane for someone I would use them and no one has ever replaced them for me. If you need one fast I can pull that one and send it to you. No problem. I'm able to get all the parts.