check out e-bay I have found tons of parts there
81hp
try the yamaha seca store.
Try one off of a yamaha seca 400
You need speciality tools and take it to a dealership that can work on it cause you can snap the crank
If it's a 1981-83 with BP7ES NGK spark Plug use a 0.032 inch gap.
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.
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.
There is NO WAY a stock Seca 2 will do 140. About 115-120, which isn't bad.
the difference is the Yamaha is better
Sure anything is possible with a little ingenuity - Yamaha parts especially older models can be swaped without too much fuss.