The mass is 50, here's the original entry for the legionnaire, try replacing your version for this one (Don't forget the square bracket):
[Ship
ids_name = 237022
ids_info = 66548
ids_info1 = 66549
ids_info2 = 66608
ids_info3 = 66550
ship_class = 0
nickname = co_fighter
LODranges = 0, 75, 120, 1000
msg_id_prefix = gcs_refer_shiparch_lfighter
mission_property = can_use_berths
type = FIGHTER
DA_archetype = Ships\pirate\pi_fighter\pi_fighter.cmp
material_library = ships\kusari\ku_playerships.mat
material_library = fx\envmapbasic.mat
envmap_material = envmapbasic
cockpit = cockpits\kusari\pi_fighter.ini
fuse = intermed_damage_smallship01, 0.000000, 600
fuse = intermed_damage_smallship02, 0.000000, 300
fuse = intermed_damage_smallship03, 0.000000, 200
max_bank_angle = 35
camera_offset = 9, 33
camera_angular_acceleration = 0.050000
camera_horizontal_turn_angle = 20
camera_vertical_turn_up_angle = 8
camera_vertical_turn_down_angle = 28
camera_turn_look_ahead_slerp_amount = 1.000000
nanobot_limit = 20
shield_battery_limit = 20
hit_pts = 2400
pilot_mesh = generic_pilot
mass = 50.000000
linear_drag = 1.000000
hold_size = 35
explosion_arch = explosion_co_fighter
surface_hit_effects = 0, small_hull_hit_light01, small_hull_hit_light02, small_hull_hit_light03
surface_hit_effects = 150, small_hull_hit_medium01, small_hull_hit_medium02, small_hull_hit_medium03
surface_hit_effects = 300, small_hull_hit_heavy01, small_hull_hit_heavy02, small_hull_hit_heavy03
steering_torque = 18000.000000, 18000.000000, 58000.000000
angular_drag = 15000.000000, 15000.000000, 35000.000000
rotation_inertia = 2800.000000, 2800.000000, 1000.000000
nudge_force = 30000.000000
strafe_force = 20000
strafe_power_usage = 2
bay_door_anim = Sc_pi_fighter
bay_doors_open_snd = cargo_doors_open
bay_doors_close_snd = cargo_doors_close
HP_bay_surface = HpBayDoor01
HP_bay_external = HpBayDoor02
shield_link = co_fighter_shield01, HpMount, HpShield01
hp_type = hp_gun_special_5, HpWeapon01, HpWeapon02
hp_type = hp_gun_special_4, HpWeapon01, HpWeapon02, HpWeapon03, HpWeapon04
hp_type = hp_gun_special_3, HpWeapon01, HpWeapon02, HpWeapon03, HpWeapon04
hp_type = hp_gun_special_2, HpWeapon01, HpWeapon02, HpWeapon03, HpWeapon04
hp_type = hp_gun_special_1, HpWeapon01, HpWeapon02, HpWeapon03, HpWeapon04
hp_type = hp_fighter_shield_special_5, HpShield01
hp_type = hp_fighter_shield_special_4, HpShield01
hp_type = hp_fighter_shield_special_3, HpShield01
hp_type = hp_fighter_shield_special_2, HpShield01
hp_type = hp_fighter_shield_special_1, HpShield01
hp_type = hp_thruster, HpThruster01
hp_type = hp_mine_dropper, HpMine01
hp_type = hp_countermeasure_dropper, HpCM01
hp_type = hp_torpedo_special_2, HpTorpedo01
HP_tractor_source = HpTractor_Source
num_exhaust_nozzles = 2