We have an Australian dealer who specialises in Maico, he has started building a very reasonably priced Maico parts
inventory since the departure of Les. Chippy has been around Maicos for a long time and is a very decent man too.
He has also listed some free on line manuals and parts lists, including a parts list for the 84SC, click below
Maico Manuals @chippy.com The number 8174 that you have is correct, that return spring being introduced for the 83 Spider engines and then continued on for decades. Although Maico part number allocations did change in 1985.
The same identical spring was then given the part number of 108174. same spring.
The reason your bike would have an alloy tank would be because most of the original plastic tanks are melted from them
coming into contact with the top of the exhaust chamber mounted below them. Often not noticed until fuel leaked over the exhaust.
I do have a pretty good original tank fitted with MXM graphics.
I also have a very good set of engine cases.
One very underrated machine that you have bought yourself. Good luck.