Somebody (xrsonly?) makes an oil temp gage for XR600's that replaces the stock dipstick, this might work for the DR350 too, since they are similar oil systems. Not sure if the threads are the same though.
For a fuel gauge, you could put a T fitting in your fuel line, and run an extra piece of clear line from that upwards along the front of the tank, up higher than the filler cap. You'd need a check valve and filter to keep dirt out and keep it from draining gas when you crash. This would let you see the fuel level in the clear hose, using it as a sight tube.
Looking at the odometer is much easier.