There are places that sell reconditioned vintage car radios - the push-button AM radios with the local stations printed on them (BL, GB, UE, KY, UW, SM etc for Sydney) - that would fit in the stock location. I don't have any links but someone else here may have them. They advertise in the classic car magazines. But they are quite expensive!
Do you really want a cassette player? The tapes are really old now, the sound is crap, and they suffer dirty heads, worn rubber drive wheels, dirt in the mechanism and chewing up the tapes. Even good tape drives suffer from 'wow' and 'flutter'. The cassettes themselves can get hot and warp if left in the car on a hot day. I know all this from experience!
Why not a modern-style radio-CD-MP3-USB but with the retro '60s look? They fit straight in a VW dash. Vintage Veedub stock them (02) 9789 1777, and I think Mick Motors in QLD does too. They are advertised in VWMA.
http://www.soundlabsgroup.com.au/p/Classic_Car_Radio/Classic+Car+Radio