126s have alloy heads so already have hardened valve seats, so in theory they should run fine on unleaded for years without the valve seat recession associated with cast iron heads.
If it was a water cooled car driven gently I wouldn't worry, but 126 valve seats were never designed for the extra heat produced by unleaded (it burns hotter) and were designed with the expectation of lead protecting the valve seats, so they might not last well without an additive. These aircooled engines run extremely hot anyway and is routinely running at 4,000rpm - gentle driving isn't really an option.
Symptoms of seat recession are hard to spot on a 126 - in theory the engine would get quieter as the valves clatter less on the seats but you'd probably never notice over all the other engine noise, and it would be a gradual change. As things got worse, you'd get back firing as the exhuast valves never closed properly - again a common problem anyway with ignition and timing, and finally in extreme cases you might have a wrecked head and pistons - a bit late then.
For that reason I'd recommend using an additive as its better safe than sorry.
Alternatively, you could have some hardened seats fitted. There's no way of knowing if this has been done already without a receipt.
