Does yours have a gas solenoid or is the valve in the torch?
Ultimately, it comes down to whether you think the base machine is worth doing the upgrade on or whether it'd be better to just upgrade the entire thing...
Seeing as you like it, euro it then.
Around 20quid for a solenoid valve, 20+ for socket (depending on where it's posted from ) and a torch for literally however much you want to spend.
When I priced up converting my SIP it was going to be around £100ish using all new components supplied from within Europe - buying direct from china would bring the price down and the postage time up, and you could get a used torch for not much money.
The only real concern I'd have is if you wanted a 5m torch and a 5kg reel - the feed motor might not like you much for that...
....as space is a bit limited in my small workshop for a trolley....