1978 was the last year for the ATC90. In 1979, the 110 took it's place so to speak - so no, you do not have a 1983 ATC90. It is possible that you have a 1983 ATC110 frame/body with a 1978 or older 90 engine in it, but they never made what you are calling it.
As for the front sprocket - I think that was Honda biggest flaw in engineering ever - having to take off the engine case / to get to the front sprocket. Good luck with it, let us know how it comes out as I will be doing this soon myself on my 1979 110. If you have more questions, this is the place to ask!
RIP - Yamahondaman!! You will never be forgotten!
RIP - Sam Brehm!! Gone but NEVER forgotten!
RIP - Sandpuppi101 - You will live on in my mind - I miss you friend!