I would suggest you start by isolating the parts you want to use personally first.
Pick a system you do like and are very familiar with as your conversion platform.
Then proceed to the basics - what is the core of how you play. If you are focused on combat, make sure you have a playable foundation, then tweak. Don't worry if it isn't perfect at first.
See if you can take some scenarios you have already played through in RIFTS (with the awkward rules) and see how the outcome was. Then try and take a similar situation from your familiar system and look at how it works, turn by turn, player by player. Don't be afraid to just choose the dice rolls - you want to know what the outcome will be to test for the variables. If your planned outcome isn't realized by your testing scenario, you need to go back and tweak it so that the outcome you desire is achieved.
It may seem a little strange, but you need to treat this like a project. You need to have objectives, and you need to have a plan. Then set small goals, day by day. For Wednesday, your goal is to write out which system you want to try to make the conversion for.
Then for Thursday/Friday, you want to write down what are the core things you want to start with. Small list. Keep it specific - such as pick one class/template/whatever and identify what are the key components of the class in RIFTS. What makes the class special, and why do you want to keep it.
If you sit down and spend some time up front, you can do this.
It gets a lot easier if you have other people helping you.
I have no interest in RIFTS, nor do I have any interest in learning it - but I am sure there are other people that do.
Maybe you could try using Google Wave to collaborate?
Just make sure you break it down into manageable chunks or you will find it very hard to get this done.
