What is needed is/are ...

What is needed is a pair of scissors.
What is needed are scissors.
What is needed is a pen.
What is needed are pens.

Are all the sentences correct?

Many thanks.

(Please note that I've re-posted because I couldn't edit. The first two sentences shouldn't have a comma.)

Yes, these sentences are all correct.

What is needed is a pair of scissors.
What is needed are scissors.
What is needed is a pen.
What is needed are pens.

They are all correct. When the subject complement in a copulative clause is a noun phrase, there is not always subject-subject complement number concord.

See this interesting article starting "There are some unusual and noteworthy points about grammatical concord in foc. wh- sentences..."

http://ling.kgw.tu-berlin.de/discourse&grammar/Text/d42111.htm

Thanks, Thomas and Contrex.

Indeed, and they didn't. They did have question marks, however, which is what I think you meant.