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I don't really get what could go wrong.
Oh I don't know, having a single bolt onto the original drop out with no way of keeping it from rotating under pressure, riding fixed without a front brakes amplified this further, and those Cannondale with the extended drop out aren't exactly known to be well made.
I don't even know of any distributor in the UK that sell them, let alone in their right mind to.
Seriously, work out the magic gearing, this is much safer.
I live in the countryside and don't ride much. This is mainly a deco bike. I don't really get what could go wrong but I understand your reserve.