The chassis number is on the crossmember behind the driver's seat (as I'm sure John meant) ... often hard to see due to paint, fluff, added carpet etc .... there's also a plate (about 2"x2") under the bonnet (riveted to the toilbox on mine but they vary ... as do lots of things with old Morgans! 🙂) ... not a modern vin-number.

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If you have the plate your chassis number would be where the red scratchings are.

Price is very subjective but not sure I'd stretch to 20k for an unwarranted and well used 4 seater in the current market ... but then I'm no expert having only ever bought one!! 🙂