Here's the easiest thing.... It may not be perfect, but it doesn't have to be. It just has to work and be close enough.
D&D is d20 base
HARP is OEd100
Therefore, for any skill related bonuses, simply do x5.
For OB, don't bother with the +20 - just use the att x5. For that dragon with +34, it becomes +170.
Keep in mind that there is a damage adjustment for the attack size that gets applied in HARP.
RR - not familiar enough with D&D rules on this, so you might need to make other adjustments.
Init is different since it is not % base - so just use +10.
-15 becomes -5
-5 becomes +5
+15 becomes +25