I have a 2007 TE-250, but can I get parts from a 2004 parts bike if needed? They look really similar in the pictures, but I wanted to make sure there wasn't some gotcha involved. When I mention parts, I mean all the way including engine.

Also....are the frames the same with the 250s and 450s? Meaning, an engine from a 450 should fit in a 250 frame?

I'm no expert Jimi, but the '04-'05 bikes were different than the redesigned models in '06. 2006 and 2007 bikes are the same, '08 a complete redesign. Some stuff will likely be the same, (like sprockets) but much if not most will be different.

my mate has an 04 te 510, i have an 07 te 450. without knowing his bike intimately i can say for sure that his forks, triple clamps, bars, shock, head, carb, tank, plastics, seat, rear master cyl.are different. the wheels look the same, as do the clutch and front brakes, radiators, bottom end, gearbox and swingarm.
others will have a better idea about what's interchangeable, but visually at least that's what i see.

paul.

The parts catalogues for various model years are downloadable so part numbers can be cross-referenced.

The head, crank, & piston are different. I think the cases are the same however.

Carb is different as are valve stem size, maybe valve head? I think a cylinder head complete would swap but not pieces

Really more interested at this point in swapping the whole engine. I'm guessing there is no reason the carb from the later bike can't be used. Not as worried about the rest. So it sounds like the engine cases are the same....maybe at least the mount points are anyway.