I'm not familiar with this engien but the pencil anode should be screwed into a brass plug which is screwed into the block. Just filing a piece of zinc to fit will not give you good enough electrical continuity for the anode to work properly. A common mistake when fitting these anodes is to put tape or sealant on the threads of the plug, which effectively insulates the anode from the block rendering it almost useless.
I have a Beta and the anodes initially supplied were a push fit/stuck in type and they tend to fall out by the end of the season. I've converted to screw in type which are obtainable from:- http://www.zincsmart.com/
In the grim and distant past when I suffered from the Beta affliction I drilled out the stub and turned new anodes out of zinc rod making them a press fit in the carrier. Absolutely no problems with function (they still got eaten away at a phenominal rate). I don't think filing some zinc down would give as good a contact between teh anode and the carrier.
Artificial Intelligence is no match for Natural Stupidity.
search on ebay - i have the same engine and found a new anode of the screw in type with refills for about £4......I think the new brass screw part and the first anode cost me about £8.......