Chris, your base looks in good nick, hope your engine is good internally.
I have a feeling this machine will be something special.
I noticed the bore size is the same for the 3.5HP engine, like on your orange rover, just a larger stroke giving 206cc compared to 147.8cc.
Could be the same piston, I haven't looked at the part numbers to compare yet.
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Happy is he who penetrates the mystery of things.