Personally, I would have thought a slightly grubby but original interior is preferable to re-upholstering in something that is going to step away from originality. Can any of the stains be removed, or improved upon with upholstery cleaner? Or perhaps keep a look out for a 90 being broken, and rob the interior from that? One things for sure, you're right about them being rare!
Tappity on tickover? Not had mine that long, and not heard others to compare, so wouldn't like to say really. Mine is slightly tappety for the first minute perhaps, when cold.
AROC Cotswold Section Treasurer
2002 156 2.0 Lusso . . .1976 Spider 2000 Veloce . . . 2009 159 JtdM Ti . . . 1985 90 2.5 V6 Gold Cloverleaf