I too have a 91 ST1100, and have made some needed changes: progressive suspension rear and front with a bit heavierfork oil. Heli-bars but I need to add a small riser as well. Volt meter cause mine still has the teeny weeny 28 amp charging system and it is easy to overtak with electrical additions. The Pan-Euro models have a stock meter that fits where the headlight adjuster is, and you can relocate that. There is often a similar part on eBay. Have a Corbin seat but I would choose a Russell as I have one on the FJR and it's the best seat I've ever had, bar none. Changed the windshield several times and types, settled on a Laminar with a Lip.
I was planning an alternator upgrade to the later model 40 amp, a couple of powerlets, Piaa lights, some highway pegs, airbox farkle and rejet with new exhaust to make the old bird breathe a little better without losing fuel economy, and mounts for some electronic additions. But the FJR has captured my heart and her 240+ range is enough for this old guy, so the ST will soon be leaving the barn to park somewhere else...