15 Engines! [X(]
Wow!! you're trying to get into the hobby in a hurry... [8D]
Rest assured that one or two of these engines will NOT live up to your expectations...
[&o]
As to the cleaning - there are many ways. it depends on the engine, the problem and the person doing the job.
If varnish (burned on blackened crud) is your problem... take the engine apart (as much as you can) and put the pieces in a crock pot filled with antifreeze (go to walmart and buy a small crock pot for ~$10) cook over night and the varnish will fall off in great chunks. Be sure to take the engine apart as much as possible and clean it thoroughly afterwards as the 'Chunks' can cause problems if not removed...
Once apart inspect all the parts for rust and damage. The bearings are the critical pieces. Look closely and be sure that they turn freely - replace ass required.
Flush out all lines, needle valves etc. the grime that can grow here WILL stop the engine at the worst possible time...
Try a search on the RCU for different cleaning tips, this is a popular subject and has been discussed many times.
Good Luck!!