Chris, as you say your S1 isn't registered so the engine only experiences slow running. You'll be surprised how it improves with ordinary work for a day or two, say 2 - 300 km at 60km/hr in hilly conditions. Everything warms up thoroughly. Keep an eye on the engine oil level - it may need a top-up after 3-4 hours. So my point is get it roadworthy and registered, and use it (that's what they were made for!) before considering a rebuild.
Cheers, Rob S