biker2013,
Read this and your other post. I am also contemplating riding the Northern tier in a couple years. My plan is similar to yours, but I plan to pedal assist at 15 mph and have the engine speed along at 15 mph for a average of 30 mph.
I think the Titan xc50s is a good engine and capable of completing the trip. It is the same engine as the Huasheng 142f.
I think you are asking too general of a question and need you to be more specific. Why such a large engine for 10 to 15 mph and not the smaller xc35? or the Huasheng 37cc from Live Fast Motors (I wouldn't buy the kits)? What type of mounting system are you going to use? What transmission are you going to use? How much is your budget for your motorized bike? What type of bicycle are you going use? Do you plan on traveling cheap (camping) or credit card (Hotel)? How many miles do you plan per day. What size fuel tank? Are you pulling a trailer?
Basically, there are 4 type of setups: 1. Friction drive - Dax, Staton and DIY 2. In-the-frame - Grubee 4G (With Oil-lite bushing bearing mod) and EZM q-matic and DIY. 3. Gearbox drive from Staton. 4. Axle drive from Staton. I don't like his particular mount and would recommend a different setup. I would weld a mount to the seat stays and chain stays. Other kits are available, but for a touring trip, I wouldn't recommend.
1. Can I make it? Sure you can
2. What spare parts should I bring? Spark plug, starter rope and maybe a spare carburetor?
3. How often should I rest the motor? I think your going at a slow pace, it shouldn't harm the engine. After break-in switch to a premium synthetic oil.
4. General Maintenance? Clean air filter. Adjust the valves and check the spark plug? Definitely oil changes for a 4 stroke engine. A 2 stroke would eliminate oil changes, but require 2 stroke oil mix every time you filled up.
As a suggestion, look at some of the other traveler post on this forum and
http://motorbicycling.com to see what others have done.
I think with a smaller engine, you'd do O.K. since you only want to go 10 - 15 mph. Set your cadence and have the engine do the rest. A right handed friction shifter makes a good cruise control.
Good Luck,
Chris
AKA: BigBlue