"I can't see someone ride it safely."
This is a sentence with a different meaning than the original sentence. The sentence implies the inability of the speaker to see someone ride the bike for some reason, for example if he is blind or in a room without windows. Although word 'see' can have the meaning of 'believe' or 'understand', in the sentence above it has the meaning of vision. You could use instead use one of the following sentences:
"I can't see that someone could ride it safely"
"I can't believe someone could ride it safely"
"It may come out to be perfectly fine."
This is an invalid English sentence. As well, 'come out' and 'turn out' have entirely different meanings from each other. 'Come out' is normally used to describe an action by something with volition, such as a person or animal. 'Turn out' in this case describes the end result of a process, such as purchasing a bike.
"It may be found perfectly fine."
This is an invalid English sentence. See answer 2C below.
"It may be found to be perfectly fine."
This is a valid English sentence. Unfortunately it has a different meaning than:
"It may turn out to be perfectly fine."
'Be found' implies an instant of discovery , and 'turn out' implies a period of time while something changes.
Also, the sentence at the beginning:
"You can't want to buy that bike."
is probably not the best sentence to use here. A sentence closer in meaning for the conversation shown might be:
"You don't want to buy that bike."
The reason using 'can't is a bad idea here is that 'can't' implies the inability to do something (have the feeling of 'want'), while 'don't' instead means that although he doesn't (have the feeling of want) it possible for him to do so.