If you're too short, you're too short, and that's the end of it. You can try for glamour agencies, which don't normally have a minimum height, or possibly agencies skewed towards the commercial end of the market. I can't recommend any though as I don't know your area.
You have a really nice look. But unfortunately, if you don't meet agencies' criteria, there's nothing you can do it about it.
The only agencies that are likely to take you on are the less reputable ones who's main aim is to take your money, too many girls are obsessed with being "agency represented" when the reality is a good percentage of those that are with agencies dont get any work anyway. Sorry if it sounds harsh but 5'1 is just too short for agency work, that doesnt mean you cant earn money on sites like this though if you promote yourself well.