the sig lt 40 is the best arf trainer you can buy!

The RCM trainer is another very good, easy to put together ARF.

Lots of good kits too, if that's what you meant by building The Great Planes PT 40, Kadet LT-40, Kadet MK III (or is it IV now?) just to name a few.

my friend totaled his plane, he sent the os 46 back to tower and the thing was so messed up he got a brand new one , its under warrenty and if you were flying with an instructor he could write a note and all your stuff will be replaced free of charge

crash damage is only covered with a very limited number of trainer combos... not all. Since the combo (if any) was not listed.... (though Goldberg trainer is sort of refferenced...) And I know of no "Success Guaranteed" combo with a Goldberg trainer, better verify that you've got the guarantee before trying to use it.

Dirt in engine:

Ususally, just pull the backplate and glowplug, and flush the lower end out thouroughly, via the carb, then flush fuel in through the glowplug location and on out the backplate.
Avoid disturbing the head screws. If you remove the head and then don't get it torqued down correctly you'll never see the end of the problems. Once you warp the head... it never seals down again.