It's hard to make a slender knothole,
If given a cord too thick.
It's hard to craft a dainty dollhouse,
If given a pyramid brick.
It's hard to make a flying machine,
If given the smallest of birds.
It's hard to lift a Broken-Heart,
Without the right words.
I think some changes could be made. Nothing truly major that would change the theme, or it could, I just don't know. However, here is my opinion:
"It's hard to make a flying machine, If given the smallest of birds."
Quite frankly, yes you can, it has been done, and man will do it again eventually. All quite easily if you think about it.
I'm also not very sure why you have capitalized Broken-Hearts, or put a dash inbetween them for that matter. As the reader, I can plainly see that the broken heart is the most important item in this poem.
I'm not going to comment on how cliché a poem about a broken heart is, ( I know somebody is going to appear and say that this is the second comment I've used with the word cliché, but that is what it is), but other than those points, it is well written, and remember:
I am most certainly not a fantastic poet. I only tread on the mediocre so my opinions might not mean much.
A short yet extremely well written piece, the way you repeated the beginning of all the lines excluding the last works very well. I've read others that have tried the same and it seems to make the piece drag and become a bore. But with your perfect structure and excellent chioce of words I really think this a piece that couldn't be altered, it's perfect the way you've written it. And all the words you offered are true to life and it seems there is no words that don't belong