For my Modern games I'm going to be using Tank on Tank from Lock 'n Load Publishing. The rules are free to download from their site here.
I'm playing it on a square grid rather than the hexagons it is intended for, and there are also a few changes I'm making to reflect the change in period from WWII.
1. When measuring on the square grid for both movement and firing half rounded up of all squares counted may be diagonal.
2. Some units are armed with ATGMs that are only effective against vehicles. These are indicated by having a second range number in brackets which they use against targets on foot. If this second range is 0 they may not fire at foot targets.
3. Instead of Anti Tank guns in the original rules I have ATGM infantry units. These may fire at no activation cost. They have a move of zero which can only be used to change facing.
4. Infantry in the same square as a transport vehicle are considered to be inside it. They may not fire while riding the vehicle and are destroyed with it if it is destroyed. They can dismount for free if the vehicle is activated for movement and are placed in any adjacent square including diagonally. This happens after the vehicle has moved, it may not move after dropping off it's infantry. HQ status can either go with the infantry unit or remain with the vehicle at the player's discretion.
5. Airstrikes use a d8 rather than a d6 for determining which squares are hit.
6. Anti Aircraft units automatically fire at airstrikes at up to double their normal range. Blocking terrain is ignored. At normal range or less the strike is driven off on a roll of 3-4 and shot down on a roll of 5-6. Beyond normal range up to double range it is driven off on a 4-5 and shot down on a roll of 6. The initial square targeted is used for determining the range. An airstrike driven off is available to be called in again on a later turn.
7. Unit Ratings.
Moggy II MBT 3-11-2
Bunny APC 3(0)-8-2
Thumper SPA 6-8-2
Woodpecker SPAA 2-10-2
ATGM Infantry - 3(0)-7/10-0
QT74 MBT 3-10-2
Bumpy IFV 3(2)-9-2
Kyuteyusha SPA 6-7-2
QT34AA SPAA 2-9-2
ATGM Infantry 3(0)-7/10-0
All bases represent platoons, and my initial battle will see a company of tanks and a company of mechanised infantry attack a reinforced company of infantry with a tank company reinforcing on turn 6. The attackers have one airstrike available.