...and I truly don't mean to sound dismissive when I say: If it's dirty, clean it. If it's worn, replace it. If it's dry, lube it... unless it's s'posed to be, then jus' enjoy it lol
It's not that I'm belittling the importance, rather hoping to illustrate the wonderful simplicity of bicycles & these motors as well - there's jus' not that much to fix. Should you encounter a specific problem however, without a doubt it's been covered in painstaking detail somewhere on the forum, as well as folks that are happy to help w/such.
Jus' for familiarity, here's an "animation of engine breakdown" vid;
...& this section is a pretty good presentation of the most common problems with bikes & motors alike;
As for ratio - we've folks that run the 16:1 suggested by the "instructions" but our best guess is that's possibly due to the rarity of actual two cycle oil in some remote regions the motor was originally designed for (1940's Russia is most likely) & jus' passed on w/the Chinese clones. The typical two stroke oil ratios are usually 32:1 with basic non synthetic, 50:1 & even 100:1 with advanced, quality synthetics... user preference prevails