Would it be okay to ask my employer to help replace my stolen bicycle?
Why would you want your employer to replace it?
Because without it I can't get work.
I understand, but it was irresponsible of you to let it get stolen in the first place. It's important to take care of your things and keep them protected, especially when they're important to you. I don't think it would be right to ask your employer to replace it.
What if I tell him it was stolen at work? I think he would replace it then.
Well, you should be safer next time, and instead of making your employer replace your stolen bicycle, you should take care of your things. A lot of people would think that you're being immature by lying to your employer.