Pre-formed bases have their drain where they want, and it is seldom where your drain currently is! Fact...
So, your choices are move it, or build a custom shower. That is a fairly small shower, and will barely be above the minimum required to meet code. Many people would find it confining. It'll work, but if you can make it a little bigger, you'd find it more pleasant.
Building a custom shower isn't all that tough, but like anything, there are critical things to ensure you get right or it will fail.
If the existing drain is quite old, it may be worth it to tear the current drain out so you can judge the overall state...it would be a pain to build a new shower using the old drain and trap and to find out that it is rotten through and leaks.
Check out www.johnbridge.com for help in building a tiled shower.