I thought I'd share my progress so far.

Instead of a Rifts conversion, I'm making more of a Rifts homage using elements borrowed from Japanese roleplaying games.

During character creation you create a 'savant' and an 'armor'.

You create savants by selecting a savant class and a number of skills from its list.
Savant classes are:
Spirals (Brave heroes that inspire allies synchronize with their armor using the power of fighting spirit!)
Holics (Doomed anti-heroes that enhance their fighting performance and synchronize with their armor through the use of designer-drugs at the cost of drastically shortened lifespan.)
Psychos (Cybernetic whack-jobs that use extensive brain surgery and cortical implants to synchronize with their armor at the cost of their sanity.)
Cyborgs (Half-men half-machines that synchronize deeply with their armors by fully integrating into them.)
Revenants (Fatalistic soldiers that use brutal training and grim determination to synchronize with their armors.)
Idols (Shining pop-stars that use the power of music to confound enemies and synchronize with their armors.)

You create armors by selecting two vehicle classes as 'forms' (you may select the same vehicle class twice). Your armor can alternate between its two forms as a major action.

Each vehicle class has a size rating (much like mutant animals in TMNT and After the Bomb).
Each vehicle class also has a capacity (according to its size) and a number of evolution points.
You can reduce a vehicle's size to gain more evolution points, or spend evolution points to increase the vehicles size. (You want your armor's forms to be roughly equal in size, as the size difference between your armor's forms is used to calculate critical fumble chance.)

You can also spend a vehicle's evolution points to buy humanoid features (just like in TMNT) to convert your vehicle into a proper mech. These features grant extra stat boosts and also give you sync points to either decrease critical fumble chance or increase critical hit chance.
Features include:
Weak or Strong AI
Grip Claws or Articulated Hands
Walker Mode or Full Upright Bipedal Mode
Semi-Humanoid Appearance or Perfect Humanoid Appearance