Need Help with Engine Diagnostics

[seventythree]

Hi Guys,

My brother is the original owner of a 2007 Boxster S with 142K miles.
Had had an engine failure at ~55K miles and had a factory crate engine installed at that time.

A couple of days ago, he got a CEL along with rough idle and a "something is broken" noise coming from the engine.

The CEL codes are:

P0503 - Vehicle speed signal
P0300 - Misfiring checksum error
P0306 - Misfiring cylinder 6

Can anyone offer any suggestion as what might be the problem?

Any & all input would be appreciated.

TIA