how to replace dustcap

I'd contact the manufacturer of the subwoofer as they should be able to get you a new one.  Depending on the design and glues used, you may be able to remove the old cap to glue on the new one.  Or you may be able to just glue the new one on top of the old one. 

If the new dust cap is polypropylene then you will want to scuff up the glueing surface to provide a good bond as it's very difficult to get a good bond on smooth polypropylene.  To handle the dust cap, it is helpful to take a piece of tape (we used 1" painters tape) and in the middle fold it back onto itself (tacky side to tacky side).  However leave the ends of the tape free and apply them to the dust cap.  This should give you a tape tab to handle the dust cap with.  To assure a good, airtight bond, be sure to apply a good amount of glue, but not to where it will squish out past the cap where you can see it when done.  When setting the cap down on the cone, twist it so the glue spreads and gets rid of any gaps.  Now just put some sort of weight on it to hold it down and let it sit for 24 hours.  Most glues will dry quickly, but take about 24 hours to fully cure. 

I always prefered a good rubber based glue as it is thicker.  But I have done many dust caps using Loctite 410.  Here's a link to the product: https://www.amazon.com/Loctite-Prism-Black-Toughened-Inst/dp/B000132VUI/ref=sr_1_2/002-0317727-1550455?ie=UTF8&s=hi&qid=1179283694&sr=1-2  Note that the picture they show is not what you will get.  But it should be more than enough for what you will need.  If you do use a CA glue like this one, you should wipe down the surface as any fingerprints or other oils will turn white from the fumes.  If any white stuff does show up, it willl usually clean off.  Good luck!