Excellent! You've got a trade background so we can move through this a bit more quickly.
The spring plates could be made as they are just literally bits of spring with a bolt hole but honestly it's probably just easier if I post you some. The bolt is just a common 1/4" bolt but I can send that too.
Good question with regard the springs on the idler pulley mount. To be honest I can't remember if there are washer etc in the mounting setup. I'll take apart the one in the photo and send you the springs and anything else that might be in the setup.
I've done a quick sketch of how the tensioner goes together. The key to its function is that the large coil spring pulls tension onto the belt and the spring plates we are discussing lock into ratchet grooves in the arm so that the pulley is locked forward once it has been pushed there by the coil spring. The ratchet mechanism has to work or you'll get belt slip.
Handling the tensioner with that coil spring can be a bit like playing with a rat trap. There is a hole in the arm with the idea that you can compress the spring and put a nail or the like in the hole to keep the spring locked compressed while you install it and set it up. Personally I never use it. I lock the spring plates away from the arm with a zip tie like I've done in the photo just wrestle it all into place
