Lots of googling and reading for you my friend.
Google: VSB14, NCOP, Vehicle Standards Bulletin
Whereabouts are you?
A VSCCS certifier will inspect / Engineering. The list of certifiers is on the relevant authorities websites, or google.
2.1 CHOICE OF REPLACEMENT ENGINE
A manufacturer’s standard or optional engine should be selected and installed using all the standard components for that vehicle model. However, where this is not practicable, the following requirements should be met:
Any replacement engine should be of similar mass and power output to that of an engine fitted by the original vehicle manufacturer as standard or optional equipment;
When the replacement engine is larger in power output than an engine offered by the vehicle manufacturer as standard or optional equipment, the vehicle must be equipped with any necessary upgrading of equipment, e.g. brakes, front suspension, etc.; and
The power and/or torque of the replacement engine must not exceed the capacity of the vehicle driveline.
Using the engine manufacturer’s published specifications, the dimensions of the selected engine should be checked against the vehicle to ensure that:
the engine, together with all of its components, fits into the available space without major frame, body or other modifications;
the engine mass and location of centre of mass must not result in excessive mass on the front or rear suspension;
the location of ancillary equipment, such as the cooling system, the intake and exhaust systems must be suitable for the vehicle layout; and
the replacement engine must be installed in a position and on an angle that allows the driveline to operate correctly.
If the installation requires modifications to the vehicle structure, such as chassis rails and firewalls, the modification must be performed in accordance with the applicable LH Code(s).