Hi Joe and congrats on a good score there mate.
Here are some Tech Specs on the Mk20 for you;
Mark 20 Villiers
Bore 63mm
Stroke 66.7mm
Capacity 206.5cc
BHP Mk 20: 2.5 hp @ 2500rpm
BHP Mk 20 HS: 2.7 hp @ 3000rpm
Valve Tappet Clearance
Intake Min .006in/Max .010in
Exhaust Min .006in/Max .010in
Points Clearance .012in / .016in
Hope this is of help mate.