The question is whether your RB250 will fit in the mounting hole and meet Rega's geometry. The RB250
is 239.5 mm long. Your current arm is 214 mm. That's a difference of 25.5 mm (or 1 full inch). The more important measurement is the distance from the arm pivot to the spindle. The RB250 needs 222 mm. You'll have to use a tape measure and see if you can get close to that number. The tonearm mounting hole on your MB-18 may be larger than the threaded bottom of the RB250. This is what I found in my DQX-500 and it gave me wiggle room to position the arm correctly. A Michell VTA sleeve
allowed me to mount the arm in a hole that was too big. It also provides VTA adjustment. You can get these on ebay.
Jeff Spall offers more detailed measurements
for RB250/300 arms:
Arm pivot to turntable spindle centre: 222.2 mm
Arm pivot to stylus (effective length): 239.5 mm
Stylus overhang (amount the stylus overhangs beyond the spindle centre): 17.3 mm
I hope this helps.