you can also use naval jelly/phosphoric acid. and having siezed a bike shouldn't increase your likelyhood you'll sieze again. in my experience mopeds are very forgiving.
also, i know this sounds like crazy bandit talk but you should try to salvage your old polini piston and rings. i've taken hard siezed (rings stuck in the grooves-piston scored as fuck) kits and completely saved them, got them running with good compression. sand the fuck out of the rings (as long as they are not snapped) and the piston and run it. i know it sounds like a terrible idea but i've seen it work well. its free try it. or if you really don't want to do that sell me your fucked rings and piston!