The lead helps the heat transfere from the valve head into the seat and thus the cylinder head itself. This is hampered by unleaded fuel, thus the valve and eventually the seat run hotter with unleaded. Damage to both can happen mostly durring hard pulling of the engine for extended periods. The correct thing to do is have hardened valve seates installed on the exhaust valves. I have had mixed results with additives. Franc is also correct in that this is a low compression engine with lower combustion preasures and temperatures. It may do OK on unleaded, unless you work it hard for extended periods. Snow cat operation would fit that bill though...
Do the valve seats with a reputable machine shop, and no worries.
Regards, Kirk