I want to do much the same.
I bought a GEBE Robin/Subaru 35cc kit from
http://www.bikeengines.com/index.htm ..once broke in it might push me up to 30mph with the 12 tooth drive gear inplace. I bought the 11 tooth for grades at elevation and the 13 tooth for long flat runs with no headwinds. With the 13 tooth I might see 34+mph and that is enough for a properly set-up MTB.
I will carry a spare belt and drive gears are a snap to change.
Search and read the posts of bamabikeguy, he does 200mile days with his bicycle. My hat is off to him...my hero.
If you need more power, look to the Tanaka 33cc or 40cc kit from GEBE.
Aux. fuel tank will keep me on the bicycle for 3+ hours. Believe me, that will be long enough. You might do cheaper and better than
http://www.sdminimotorsports.com/site/707069/product/Tank 1 This will be mounted between the engine and seat post.
Carrying gear?? My BOB Ibex, with all options, will be here on Tuesday. Best deal ATM is $339 w/free ship and no tax from
http://www.triplesportsonline.com/
The BOB Yak is only $299 with all the options but I want suspension so the 16" tire can survive pot holes and such. Might do a little single track inorder to free camp.
If you go GEBE, consider the 12g spoked rear wheel they offer fairly cheap at about $100 shipped. They will fit the drive sheave and it has the solid axel installed.
If you decide on a BOB, be sure and order 10x1 BOB Nutz so it can be mounted to the rear axel as the QR will not work...bad idea to use a QR with powered bikes.
Carrying water??
http://biketrailershop.com/catalog/index.php > Racks for Bikes/Trailers...top of the page, $55. Water bags can be slung over this rack like saddle bags. I have yet to order one but will do so soon.
Need cool water w/o ice or refidgeration? Consider two of these flax water bags. I have yet to order three but will do so soon. (Why three? Trading on the road or to return a favor of help on the road).
http://www.lifestylestore.com/ls_survival_water.html
There are solar bags that hold considerable water for warm quick showers. I'm sure you know of those.
For some real insperation.
http://www.mountainbike-expedition-team.de/
If you have never toured in hot weather on a motorcycle, w/o a windshield as I have, you need to be aware of several things. You must be in decent shape, you must stay hydrated... coffee, tabaca, alkyhol and lots of sugars are a no-no, stay covered with long sleeve shirts and pants, rest often, use sun block.
OH!
http://www.staton-inc.com/ offers some very road worthy kits and be sure to look at their uber-heavy duty rear wheels. The hubs they offer have 4...yes,FOUR cartridge bearings and will last forever on a bicycle. Overkill at its best.