Are you sure you're getting spark on both plugs? The YT16s ran a Briggs 402707 opposed-twin. It has a unique ignition, in that both spark plugs run off the same magneto with two seperate spark plug leads (It's called a "lost spark" system). It's not uncommon for one side to fire and the other not, even though they're running off the same mag/timing point. If the compression is down a little, it won't start if it's firing on just one side. Check the manuals section of this forum, I'm pretty sure I've posted the Factory Service Manual for the "Briggs Opposed Twins". Of all the Ford Lawn Tractors, the YT16 are the most sought after by collectors because that opposed twin is one of the best engines Briggs ever made. The tractor was actually made by Gilson and they built great machines. You have something worth the effort to get running. That magneto can be bought aftermarket for around $20
Amazon.com : The ROP Shop Compatible Ignition Coil Replacement for Briggs & Stratton 402707 402777 404417 404437 404447 : Garden & Outdoor
They built two different versions of the YT16. Yours came with a 402707 engine and had a Peerless 5-1 gear transmission. The YT16
H came with a 402707 engine, but had an Eaton hydrostatic transmission. Eaton 700 from 84-89, Eaton 850 from 89-91. The 89-91's with Eaton 850 are considered the best by collectors.
Noticed no deck in the photo.... Don't recall on the YT16's, but the earlier Ford GT's came with aluminum decks. They were prone to cracking and have long since been sold for scrap weight. A working deck for the Ford 100's, 120's, and 140's can go for $300+ if you can find one