Change this line in the Transportation rule:-

If the target Territory is controlled by another Country, it cannot be moved to.

To:-

If the target Territory is controlled by another Country, this is called Invasion Transportation (or simply Invasion). Invasion lasts an indefinite number of periods (use ‘?’ to represent the number of periods in the “In Transit” section). In order to make an attack post at a Territory controlled by another Country, a Country must have a population presence in a Territory adjacent to the one being Invaded of at least the same amount as the number of units being moved in the Invasion, and must pay for a period of Invasion Transportation there, which costs the same as Hostile Transportation plus 1 Metal per unit. If the defending Country loses control of the Territory, the attacker may reduce the number of periods of that Invasion Transportation to 0 and thereby complete the Invasion.

And add this to the Conflict rule:-

In the case of an attack method used in the course of an Invasion, the population units being moved in that Invasion Transportation is considered the attacking presence in the Territory.

In Construction, Change the first bulleted effect of Fortification to:

The effective size of an attacking presence in a Territory in which the defender has a Fortification is halved, rounding up, when calculating the results of its attack.

And also change the second sentence of the first bulleted effect of Cannon to:-

If e does, the effective size of the attacking presence will be multiplied by 0.75 and rounded up instead of being halved, when calculating the results of its attack.