How do you solve the Kepler Problem for a 2-body system where m1<<m2 with a given velocity and position vector resulting in a new velocity and position for a certain time interval?
There are many ways of solving Kepler's equation - hundreds of them. Any decent celestial mechanics text should present several. Have you searched around on the web?
In general, if you're going to do much celestial computing, I recommend the works of Jean Meeus.