the difference is the Yamaha is better
try the yamaha seca store.
Try one off of a yamaha seca 400
The Yamaha XJ750J, also known as the Maxim, is a cruiser whereas the XJ750RJ, the SECA, has more of an European racer style. They have the same engine but have a different gas tank, body panels and seat.
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
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.
Sure anything is possible with a little ingenuity - Yamaha parts especially older models can be swaped without too much fuss.
Maxim 400
I have a 92 seca 2 with a 600cc motor, and it takes 2.6 quarts of oil to fill it up.