Its difficult to maintain type with marked mice as obviously you primarily select the mice which are the best marked whereas with self its slightly easier to look at ears, tails and size etc.
I think broken is recessive so if you outcross to a self which has got huge ears you will lose the markings but if you look at the long term you can then pile the recessive broken gene back in and you hopefully will gradually get the markings back and keep the type.
Alternatively you could try to find another breeder who has brokens with larger ears whos willing to give one up!
Probably best to do both. I dont know much about brokens and dont knwo anyone who breeds them, so you might find it easier to develop your own lines than find a breeder to let you have one of theirs but good luck with it
Both selective breeding or using a good outcross will work. However if you're starting off with pet quality brokens the easiest thing to do would be to start with only show brokens and not use the pet ones at all. There are a few breeders of brokens about including sarahc on this forum. If you want to play about with them because they're for pets only and you're not worried about markings then you could breed any mouse with big ears to your brokens, then breed the siblings together to get the broken appearance back.