Remove the body (important) then a solvent to clean the rest. Depending on the age- the grease was likely a clay-based one- the newer greases will not cake up like clay-based greases did.
First, remove the really hardened "chunks" stuff w/ a small screwdriver. Start with mineral spirits and a brush, dipped into the MS. That should work.
Harsher chemicals (containing acetone, lacquer thinner etc.) and contact or electrical cleaner might do some damage to any plastic parts. "Plastic Safe" cleaners may work well, but depending on chemical makeup might also do some damage.
Re-lube gears with any of a variety of greases- all should work well- from fishing reel grease, to red or white lithium greases, to Lionel 6-62927-L Multi Purpose Lubricant, to Labelle 106.
