Welcome to the Forum! You'll find some interesting people here - rather strange characters, in many cases, but generally harmless.
We've been seeing quite a few complaints of this nature lately. It seems Heller's quality control isn't what it used to be.
You really have two basic options: try to straighten the parts, or replace them.
Styrene varies considerably in its flexibility. The highest quality stuff can take quite a bit of very gentle flexing, applied carefully with the fingers, without breaking. We've been hearing lately, though, that Heller's been using some inferior, rather brittle plastic that doesn't respond well to such treatment.
Submerging the parts in hot water, or warming them genty with a hair dryer, sometimes helps. (If you try the hair dryer trick - be careful. It's easy to overdo it, and find yourself holding a molten blob of plastic.
If I remember correctly (a highly dubious proposition these days), the topmasts and topsail yards are molded in halves. If so, you might be able to straighten them by sandwiching a piece of stiff wire, held in place with epoxy or superglue, between the halves.
If you do break a spar, all probably is not lost. Using a pin vise or a rotary tool (e.g., Dremel, operating at a very slow speed) drill a hole in the broken end of each fragment. Make the hole fairly deep - at least 1/8". Then insert a piece of wire, cut to length in advance, into one of the holes. Make the wire a little smaller in diameter than the hole, to allow for any errors in the location of the hole. Put a drop of slow-drying superglue on the end, and smush the two broken parts together. If you do that trick carefully, the repaired break will actually be stronger than the rest of the spar.
The obvious alternative is to replace the whole spar with one turned from wood. In the case of a topsail yard, that's rather challenging; if you look at the plastic part carefully you'll see that it's actually a rather complicated geometric shape (which, incidentally, Heller reproduced very well). The topgallant yards are quite a bit simpler in shape, but still not simple tapered cylinders. If you do find it necessary to go that route, a good wood for the purpose is birch. The typical American hardware store or home center generally sells dowels made of three species (and frequently doesn't bother telling the customer which is which). Avoid oak if possible; it's got a coarse grain that's tough to finish. Maple is nice, but tends to be a bit brittle sometimes. Of the three, birch is probably the best - as long as you pick out a dowel that's been cut so the grain runs parallel to the length. (If the grain runs across the length, put it back in the rack. It'll warp.) The best dowels of all, though, are cherry. Cherry dowels are pretty scarce in retail outlets, but can be ordered from woodworking supply companies (e.g., Lee Valley: www.leevalley.com or Woodcraft: www.woodcraft.com ). If you've got a lathe, you already know what to do next. If not, you can get remarkably good results, with practice, by chucking a length of dowel into an electric drill and going to work on it with increasingly fine grades of sandpaper.
Good luck. It's a great hobby.