The failure to start has nothing to do with the belt.

If the hose came out of the purge while the engine was running, then you have introduced air into the system. Unlike the 617, this engine does not give you the pleasure of a hand pump.

So, crank away. But, give the poor starter a rest every 30 seconds. Allow it to cool for five minutes or so. You might need a battery charger if you sucked up some serious air.

Even when the secondary is changed on the 603, and the new filter is filled, the engine gives a bit of trouble and stalls a couple of times due to the air introduced.

It can be a PITA if there is serious air in it.

As BB mentioned, you can use ether to clean the oil off the belt. It takes quite a bit of it due to the very volatile nature of the ether. I'm sure you could use alcohol and get similar results because you have much more liquid to work with.