I would love to work as a motorcycle mechanic myself.
I work full time as a motorcycle mechanic and yes, its a dream job if you are into bikes like I am.
There is a major "but" though.... If you are employed or own a repair shop the business side of things take over regarding what type of work you take in. Our shop in london is always busy and there is no way we would take in a gearbox job as its not cost effective for the shop or the customer. yes I could do the job but in the case of our friend with gear box issues its easier and cheaper to swap the motor.
There are mechanics who own businesses out there that specialise in specific types of mechanics but tend to charge a lot more as its a time consuming job. Bikes are getting more reliable these days and you would be rather unlucky to encounter a gearbox issue, this is why jobs like that are very rare in many bike dealerships or repair shops plus most people would shit themselves over the repair cost.
As a bike mechanic you're brain has to take over as to what you can achieve compared to what you really would like to do as far as the job is concerned and often this means condeming customers bikes because its not cost effective to fix.
To be honest its a great job and I love it but has many ups and downs like most jobs, if you are looking at it full time good luck to you . :