I translated some

of the regulations and restrictions found on the Finnish Vehicle Administration web site.
You can find it in
Finnish here. The English version is pretty much useless
So, regarding the vehicle
engine change:
It can be done with several restrictions and requirements.
If a model series of a vehicle has several engine options, the new engine is compared to the engine with the highest values of the property being compared to (Eg. horsepower)
The new engine cannot weigh more than 20% more than the compared engine. If added weight is more than 10% than the heaviest comparable engine, you must compensate added weight on the axle by some way (eg. move battery or lessen the maximum number of transportable people)
The engine cannot be more that 20% more powerful than the most powerful comparable engine.
The cylinder capacity cannot exceed more than 20% of the biggest comparable engine.
The brakes and power train must always be modified to match the vehicle equipped with the compared engine ( eg. sj410 needs brake upgrade and antiroll bar, when installing a suitable 1.3l engine)
Also all modified mounting points need to have appropriate welds.
Engine change requires the vehicle alteration inspection to be street legal.
Also vehicles taken to use before 1.7.87 only need to meet the emission standards of their time.
Vehicles taken to use 1.7.89-31.12.91 need to meet the ECE R15-04 emission standards.
Also a pile of paperwork regarding the engine and the swap are needed.
Altering the original engine
Non-turbo engine mods allowed:
-Carburetor change
-Installation of fuel injection to a engine with a carburetor (this is automatically calculated as a 10% power gain, unless proven otherwise)
-Alteration of ignition system
-Change of engine compression ratio
-Change of cam shaft
-Alteration of cylinder head
-Change of intake manifold and exhaust manifold
These modifications don't require an alteration inspection
The following modifications are compared to engine change
-Any alteration of a turbo engine
-The installation of an intercooler or a turbo
-Change of cylinder head
-The increase of engine cylinder capacity more than 10% (power is calculated to rise at same rate as cylinder capacity rises, unless proven otherwise)
These modifications require alteration inspection of the vehicle
Tires and rims
No alteration inspection required IF:
-Diameter change is less than 26mm
-Width 30mm
Alteration inspection required in change is from the listed original size
-Diameter change MAX 51mm
-Width MAX 102mm
-Vehicle track width change is MAX 30mm
Axles
The new axles have min. the same weight classification as originals
All axle parts other than mounts need to be OE equipment
Track width change MAX 100mm
Axle linkage needs to be mounted securely and with enough strength
During alteration inspection you must give a written explanation of the weld additives and welding job and the strength of any self-made mounting points.
All welds need to be unaltered during inspection(No grinding welds flat)
Bodywork
Side windows can't be replaced with plexy
It is allowed to change the number of doors within the vehicle model range(eg. 2door to 4 door)
body needs to be reinforced to meet the strength equivalent of the model with the same number of doors
Body width increase MAX 100mm compared to original vehicle width
Flip bonnet allowed
Body lift allowed for vehicles weighing over 1500kg
-Lift can be MAX 77mm
-It is forbidden to remove original body mounting pads (Needs to be included with the lift)
-
Total vehicle height increase with body lift/suspension lift/tire size increase CANNOT exceed 150mm
- Lifted open tops need a roll cage installed
Chassis
Suspension lift in a 4x4 is MAX 77mm
Suspension lift cannot affect significantly to the camber and caster of the wheels
If lift affects angles significantly, lift is forbidden
Yeah, so that's pretty much it.

There are some more technicalities I didn't translate, but I hope this is enough.
Also if a mod isn't listed, it probably requires alteration inspection like the listed, or is forbidden.
People can point out things I've misread or otherwise translated bad.