There's a bit of plastic on my LK3054 - the cowl around the instruments, the grab handles on the fenders, and the grill area. Not much, and I like that about it. Given the simple shapes, I could fabricate something out of metal to replace it if parts were not available.
But what electronics are you referring to? Relays ain't electronics, there about as high tech as hammers. There may be some electronics in the guages on my LK, but I wouldn't bet on it. There's a switch on the brake for the lights, and one on the clutch for starting - none of them are gonna disable it. Other than some of the JDs with an electronic control system over the hydro system, (which I would never buy, & I'm an electrical engineer), and some of the guage panels I've seen on Kubotas, I don't think there are any electronics on most any of these machines. Maybe an electronic voltage reg in the alternator - which would be a welcome improvement over the relay style regulators on the old tractors! Any blacksmith worth a darn could coble up a fix for a switch or relay! Heck the tractor switch I most worry about is the starter switch in the gearbox of my old Ferguson, mostly because it'll be a real pain to change.
Anyway, while I personally would stay away from a tractor with a lot of plastic parts, there are planty of new small tractors that are mostly metal, simple, and likely to last as long as my old Fergy.