I added a new-rig spinnaker to my 1971 Panthercraft Tornado, and have had no problems, including regular bow stuffing with spinnaker and upwind in 6' wind waves (not including swell).
I've also sailed with a new rig main in addition, with no problem.

I know of at least 3 converted Sail Craft of Canada Tornados and they've had no significant structural problems, but Geoff Dobbs described (in the TornadoCat archives) how they did improve the SailCraft structure over the years.

I highly recommend the conversion. You can get real deals on used new-rig sails. I've only got $2000 invested in my boat (including tilt trailer) and you just can't beat the performance (or fun) per dollar. (And there is a $100 T for sale in Gardena, California with tilt trailer at the moment.)

Yes, the old boats don't compare to the new in terms of stiffness, but they are just as fun if you don't mind doing a little work on them. And they are a lot cheaper than a new Marstrom at ~$30000 USD without trailer or sails.

--Glenn (TornadoCat list member)