The Filter is inside the round cap at the lower front of the motor. The drain plug is right below that. I usually remove the filter first and the remove the drain plug
Maxim 400
82.00 HP (59.9 kW)) @ 9000 RPM
my manual for '82 seca 400 says "Periodic oil change: 2.2 liters / 2.3 us quart .... With oil filter replacement 2.5 liters / 2.6 us quart"
try the yamaha seca store.
Try one off of a yamaha seca 400
If there is one it will be located from the main fuel line running from the petcock to the carbs.
ebay
Yes, a round headlight will fit on a Yamaha Seca. Just be sure that the model and specifications match what will fit into the mounting brackets.
81hp
You need speciality tools and take it to a dealership that can work on it cause you can snap the crank
2
The spark plug gap in a Yamaha Seca is 1/2. It can be easily obtained and is very affordable and likely to be replaced by the owner. It is a quick replacement.