Heres the engine specs on both the engines you listed.
CVRX 15
Bore: .591" (15mm)
Stroke: .555" (14.1mm)
Weight: 8.47 ozs. (241g)
Practical RPM range: 3,000-33,000 RPM
Output: 1.2 BHP at 28,000 RPM
Crankshaft thread size: this is an integrated pilot shaft- 5mm thread
Mounting holes Center/Center across front of engine-37mm (1.459")
Mounting holes Center/Center each side- 14mm (0.552)
Front of Crankcase to Rear of Crankcase- 60mm (2.365)
Bottom of Crankcase to Top of Cylinder Head- 84.6mm (3.331")
Width of Crankcase without mounting flanges- 29mm
(1.145")
CVRX 12
Displacement: 0.129 cu in (2.11cc)
Weight: 258g
Bore: 0.55" (14.0mm)
Stroke: 0.54" (13.7mm)
Power Output: 1.0 bhp / 31,000 rpm
Practical RPM: 4,000 - 36,000
Crankshaft Thread: 5mm
On-Center Mounting Holes, Front to Rear: 11mmm
On-Center Mounting Holes, Side to Side: 31.4mmm
On-Center Exhaust Mounting Holes: 27.5mm
Back of engine to front of drive washer: 2.23" (56mm)
Height: 3.25" (83.7mm)