I never played this board game but it seems that it is a ripoff from this video game also called "Lost Patrol":
https://www.youtube.com/watch?v=Gy5bOEF14WM
American soldiers survive a heli crash in Vietnam and have to reach the nearest base on foot while moving through enemy territory. Great soundtrack and terrific game overall. I liked to play it back then. But now back to your custom rules. Here are my suggestions:
Tangleweed rolls
A knife provides a bonus of +1 but you should give the chainsword a +2 instead because it is a bigger weapon after all. It is also an iconic weapon of
40K so give it some love.
Shotgun
+1 bonus to shoot Tyranids on next tile? It´s a close ranged weapon, so it wouldn´t make sense to give it a bonus for fighting at a distance. Provide a bonus for fighting on the same tile and give it a bonus to the assault score because it is an assault weapon in
40K.
Bolt pistol
As a pistol it is available for assaults, too. So give it a bonus in that department as well.
Heavy Bolter
Should have a bonus for shooting up bugs on adjacent tiles. It is a heavy weapon after all. Here, see a scene from one of my favourite movies:
https://www.youtube.com/watch?v=wgzxSr6l9Y4
Sniper rifle & Missile launcher & Frag & Krak grenades & Auspex
Scouts have access to these weapons/wargear as well. Why didn´t you write rules for these too?
Remember that this game is even harder than Space Hulk. I have seen youtube videos of people playing it and they reported never been able to win at this game while playing the Scouts. So they deserve every conceivable help you can think of.
Coming to think of help, why don´t you take a poker deck and write special rules for all the cards. This deck would be used as a random events deck in which one card is drawn per turn by the Scouts player at the beginning of his turns. He can hold as many or as few of these cards in his hand as you would deem necessary. These events should be beneficial to the Scouts player most at the time because the game is already really tough. Here are a few suggestions of mine:
Ace of Spades / He is behind you
Play this card at any time, apply it´s effect and discard it.
Put a model of Sly Marbo on any tile of your choosing and remove all Tyranids & spawn counters on that tile as well. At the end of your turn roll a
D8:
1-6: Designate six different directions on your hexagonal tile on which Sly is located before rolling the
D8: Move Sly Marbo up to the next adjacent tile. If there is no adjacent tile then remove Sly from the game. If there are Tyranids & spawn counters on the tile to which he just moved to then remove them now as well.
7: Sly stays on the tile.
8: Sly is removed from the game.
Sly Marbo is war incarnate. He can´t be attacked by Tyranids and every Tyranid which ends it´s movement on the same tile as Sly is immediately removed as well. What is Sly Marbo doing on this Emperor forsaken world? Holiday, what else?
Queen of Hearts / Copulating Season
Play this card in any of your turns, apply it´s effect and discard it.
Choose a tile on the board which has either Tyranids on it or a spawn marker. Choose one of two possible effects:
A) The Tyranids can´t move in their next turn.
B) Remove the spawn marker.
Ten of diamonds / Rainfall
Play this card immediately, apply it´s effect and keep it as a reminder beside the gaming board. All shooting at adjacent tiles suffers a malus of 1. Also whenever a Scout or Tyranid or a group consisting of said gaming pieces wants to move to an adjacent tile, which is titled as the destination tile, roll a
D6:
1-2: The model(s) are moved to an adjacent tile which is to the left of the destination tile. If there is no such tile than the model(s) are not allowed to move at all.
3-4: The model(s) are moved to the destination tile as planned.
5-6: The model(s) are moved to an adjacent tile which is to right of the destination tile. If there is no such tile than the model(s) are not allowed to move at all.
At the end of your (Scouts) turn, roll a
D6: On a 6+ this card is removed from play. Otherwise it stays in play but put a marker on it. At the end of the next turn (Tyranids) roll again a
D6 but apply a bonus of 1 for each marker which has been placed on the card. On a 6+ this card is removed from play as usual and otherwise place a marker on it as well.
2 of Clubs / Agent Orange
Play this card at any time, apply it´s effect and discard it.
Exchange a Tangleweed tile with a randomly drawn new tile. This new tile can´t be another Tangleweed tile nor the Spaceship tile. Simply draw another card from the deck. If a Tanglweed or the Spaceship tile is drawn, put it/them back into the deck and shuffle it.
5 of Spades / Contact!
Play this card at any time, apply it´s effect and discard it.
All ranged combat performed by the Scouts in this turn gain a bonus of +3 but the Scouts may not move in the same turn as they have to reload their guns.
I could go on but it is your game after all so if you like this idea you can come up with suitable ideas on your own. Watch this game in the link for inspiration for other encounters/effects.