You can, and the easiest way of doing that is by buying a new rear wheel with an internally geared hub. You'd need the shifter too of course. It's possible to replace the hub in your current wheel as well, but with labor and perhaps new spokes that'd probably be more expensive.
Talking about riding speeds and riding distances on a stationary bike is rather pointless because the bike isn't going anywhere, at any speed. If you're riding for exercise, count the time instead.
Yes you can put on a 110cc bike engine on a 50cc bike and if there is any modification work then just take it to a local bike repair shop.
Any bike with pedals.
The word BIKE in this question is a little vague. Obviously a bicycle can be rode on any road without a permit.
ride it without any clothes on
You must have to concern your mechanic for this problem you should never do this without under any supervision of expert. it may effect on the performance of your bike.
Whether or not a bike trader is a worthwhile hobby depends on the individual. A person without any interest in bike trading would not find this a worthwhile hobby. However, a motorcycle, dirt bike, or uni-bike enthusiast might find bike trading to be incredibly worthwhile.
they are OK, but not for professional needs. But you can use it long time, without problems!
Not just 125...Any bike in general. However not all parts, and even those that do, usually don't add all that much. However, if you have lots of money and can find good parts you can add about a third of the current power to your bike.
There are no e's in that! =D
Unicycles. But any bike can be ridden without hands as long as you can keep your balance.
cycle as slow as you can without falling off to a finishing line.