According to a German site I've found there is a K & N filter for the
Civic 1.8 (referred to as FK2). Plus a few other goodies including carbon-fibre look wing mirrors and a replacement 1.4 engine cover!!
K & N themselves have an
FAQ on the subject of
MAF sensor failure.
All told, they reckon you get a max of a 4 bhp increase, but even that varies according to driving style etc.