What exactly did they tell you, why it cannot be rebuild? Usually, if the engine needs bored, it can be done 2 or 3 times if oversize pistons are available.
If the first rebuild required a substantial size overbore due to a catastrophic failure, you may be at the limit.
Difficult to comment without a tad more detail. I would think that short blocks for the 230SL should not be that difficult to come by. The M127 was a fairly common motor.