What size are the correct fittings for 1 1/4" ABS pipe?
They definitely exist (in Canada at least).

Sample "big box" listing for "abs 1 1/4" :

https://www.rona.ca/webapp/wcs/stores/servlet/RonaAjaxCatalogSearchView?catalogId=10051&amp;searchKey=RonaEN&amp;urlLangId=-1&amp;content=&amp;langId=-1&amp;keywords=abs+1%22+1%2F4&amp;storeId=10151