The bikinitop in the eBay link is a good one! Bonytops is the only one who makers new tops for the LJ80 at the moment as far as i know.
http://www.bonytops.com/
Doors are rare at the moment. A friend of me have many doors, but i think shipping will kill you... if he's willing to sell.
If you need a color code of an original color, check here:
http://www.offroadrunner.nl/kleuren-lj80.html
Engine: the original 800 is strong enough! I drove my old 800 for 15 years without any problem, and it brings me all over Europe!
Although, when you decide to use a new engine, a 1000 is allways better! But if you have the opportunity, take the engine out of a Suzuki SC100!
Original 800 = 39 hp
1000 out of the 410 = 45 hp
1000 out of the SC100 = 49 hp!