/home/pi/.dgen/dgenrcto add the 2xscaler command. You may want to give this a try instead of the 2xscaler though. I was able to achieve a slightly higher framerate using these settings.
# Run fullscreen?
bool_fullscreen = no
# If you want to increase the size of the window, increase this value.
# It currently must be a whole number.
int_info_height = -1
int_width = -1
int_height = -1
int_scale = -1
int_scale_x = -1
int_scale_y = -1
int_depth = 0
bool_swab = false
bool_opengl = true
bool_opengl_aspect = true
bool_opengl_linear = true
bool_opengl_32bit = true
bool_opengl_swap = false
bool_opengl_square = false
bool_doublebuffer = true
bool_screen_thread = false
# Use a joystick?
bool_joystick = yes
# Use OpenGL mode?
bool_opengl = no
# Set these to the resolution you want to run OpenGL mode in.
int_opengl_width = -1
int_opengl_height = -1
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