Nice find! Old reels like that are pretty cool. I did a quick search using the name Heineke Co. Springfield IL and found that owners manual. I think it covers both the model 518 like yours and the 521. If you can find the numbers on the engine you can probably find info on Briggs and Stratton’s site.
Thanks. I found some other lower quality pdf's but still are unsure if this covers the 521 and 518. Going to pull the shroud next to see if I can find any stamped numbers.
Came across this thread on another site that looks like mine. Notice the arm that attaches the handle to the motor is different and what it came with, although not attached. Fella did an awesome job. I don't ever expect to get there, but do want to get her running.
I had one of those real mowers when I was a kid back in the 60's it was orange I believe it was a Jacobsen. I fixed it up and sold it to a neighbor. I wish I still had that but that was along time ago.