Would it make sense to take an engine that is run down and beatin up and put new parts into it like piston sleeves and all of that stuff or is it just cheaper and easier to buy another engine from the start?

     I ask because i also have a radio controlled nitro car and when my engine wears out i just buy a new sleeve and piston Which makes it run like new!

I guess it depends on the engine...  Some engines cost more to fix than they are worth.

Engines is cheap these days.

Like the guys say, it depends on the engine.  If it is a ball bearing engine, then you can usually just swap a new piston/sleeve in it and get a "new engine".  *Most* sleeve piston sets that are still available from the manufacturer will be cheaper than a whole new engine. 

If it's a plain bearing ("bushing" in the case like a Fox 35), then bushing wear *might be a problem*.  Then you got to make sure the rod is ok.  A lot depends on how well te engine was treated.

Do you have a specific engine in mind?