The best estimate we can make of the age of an SB45 is to use the manufacture date of the Briggs and Stratton engine and add three months or so. Your mower's Briggs has been replaced by a Tecumseh (a remarkably bad decision by somebody) so we can't estimate the mower's date of manufacture except from the fact that it is a one-piece-deck model and therefore an early example of SB45. Deejay will be able to tell you what that indicates in terms of years.
From your report, your mower was in average or better, condition for an SB45 of its age. The faults you found are normal, and should be expected in a second-hand SB45, even if it were a somewhat more recent model. Because you intend to replace the engine anyway, with a GX160 which is very much better engine if it is in normal condition, I suggest you don't waste your time doing anything to that Tecumseh beyond spitting on it in passing. The rest of the work in front of you should be straightforward, and largely covered by Deejay and Joe's videos. Hopefully you'll find it fun, and be very happy indeed to end up with a nice example of a one-piece-deck, 20", OHV Honda-engined, SB45. I suggest however that you post some better pictures of the mower that accurately show the condition of the paint. Repainting an early SB45 just to make it shinier, sounds like a move that would set the SB45 collectors baying for your blood.
