bitubo shocks are supposed to be pretty good, but expensive. yr cheapest source will be s.i.p. in germany , but you can get them here in the us for about $230/pr. in addition, s.i.p. makes a shortened bitubo for the front, which would allow you to either run a shortened p fork in an older bike to keep the same ground clearance and rake, or swap the short shock into a stock p front end for a slammed-front, dragster look.
sebacs are another option, but i’ve heard they’re pretty unreliable. feel free to disagree, kids. anyway, they’re about $50-90 apiece, cheaper if you order from germany. the back shock lets you adjust the preload for carrying a passenger or a lot of cargo.
another option that i’ve used is some stiffer springs, keeping the stock dampers which are pretty hard to kill if you get genuine piaggio. the shock units disassemble pretty easily, and from there you can swap in new springs. i got mine from two-wheeled engineering in the UK; you can also get them from stoffi’s garage in austria , or get stoffi’s springs from s.i.p. as well. i like em a lot, but the rebound is really undamped (the stock dampers were never designed to handle these springs). there is no sag once the bike is off the stand and maybe 1/2 in. w/ my 200 lbs on the seat. they are strong and improve cornering a whole lot, i think. also i can smack potholes with abandon, even two-up.
stoffi’s garage in austria also sells stiffer springs for pre-p series bikes (sprint, rally, super, etc) that are available in chrome! this, as far as i know, is really yr only option for an older bike.