Help please - motor knocking sound in new-to-me '09 R3T

Its sounds to me like you have a good handle on her. Yes you need the slug. And youcan look in the hole and see if it has the plug or not. First in the picture above you can see a small black circle down in the bottom of the plunger. Smaller then the actual plunger inside diameter. It is a pre drill point under sized to remove material prior to the flat bottoming machine work for the spring to rest in. If the tensioner has a spacer/slung in it it will not have be thay drill point mark as the slug will block the view amd have a even machined flat surface . Hopefully my grenades have not screwed my spelling up or my ability to explain if you find you accidently stuffed a spacer in her and she already had one a pencil or rid with a dab of grease or beeswax will help you retrieve it. Do not be afraid to lean the bike over slowly it does help if grease does not work. Tensioner from my memory jas about 23 to 25 mm drop and ypu done without midifications.