X-Force Genesis

Magnets is under the upgrade menu at the title screen. You'll need about 7,500 to fully upgrade Magnets to max. (1,500 x 5 upgrades)
Just playing through the game and killing enemy ships will give you enough. For me Stage 4 was the best. Enemies are plentiful and the end boss is fairly easy. I get about 3,500 to 3,800 for completing this stage depending on number of enemy kills and of course killing the boss.