You might want to do without a center until you find a good match. Unfortunately, it is hard to know what will match what you have, other than another Canton Quinto 510, which would be a perfect match for your front right and left speakers.
Speaker manufacturers change their designs over the years, so it is very hard to know if anything current would really be a good match or not (though a manufacturer is almost certain to try to sell you something regardless). If you can find another Canton that uses the same tweeter and midrange as your Quinto 510 speakers, that would likely be a good match.
Canton made some good speakers over the years, so I imagine your Quinto 510 speakers are pretty good. So don't be too quick to replace them with something else just so that you can have matching speakers up front. Make sure you like them more than the Quinto 510 speakers or just do a "phantom" center (i.e., do without a center). And, of course, replacing all three up front with something better would cost you more than $200 anyway.