If you just inserted that code into the code above it will not work because in this line,
TranslateEntity ship,x_vel,y_vel+GRAVITY,z_vel
x_vel,y_vel+GRAVITY,z_vel is saying
TranslateEntity ship,0,gravity,0. So you need to update those variables somehow. Also in your code above, you do realize you are moving backwards? Of course thats beside the point.
Where you have If keydown 200 make it something like
if keydown 200
y_vel= y_vel + speed
end if
Also I think you should scale down everything first, just to count that out as a potential problem.
Good luck.
TranslateEntity ship,x_vel,y_vel+GRAVITY,z_vel
x_vel,y_vel+GRAVITY,z_vel is saying
TranslateEntity ship,0,gravity,0. So you need to update those variables somehow. Also in your code above, you do realize you are moving backwards? Of course thats beside the point.
Where you have If keydown 200 make it something like
if keydown 200
y_vel= y_vel + speed
end if
Also I think you should scale down everything first, just to count that out as a potential problem.
Good luck.