It does look to be fairly lightweight so I'd have to say that it would only suit a light(ish) bike. Having said that, it looks more robust than some trials chairs you see. The seller says it was fitted to a pre-65 trials bike and that may well have been a big thumper.
Another thing is, as Jethro will testify, it's easier to strengthen a sidecar you already have than to fab one from scratch. My first thoughts would be to lace up the wheel with HD spokes (if it looks like it needs it) and to see if the swingarm needs some bracing. It's a certainty that the shock will need to be beefier, but a trip to the breakers should get a shock for very little wonga.
You would have to pay some attention to the fittings, especially as you will need some means of adjustment rather then bolting it rigidly to the bike frame (as it would have been originally). But nothing's impossible with a welder and an angle grinder.
If you get it sorted it will be an attractive outfit.
GC
