The sentence in query can be translated into contemporary Japanese as below:
もしも小説家が事実を得て満足すれば、どこに天来の妙想を付けることができるだろうか？

Ogai is criticising "literary realism" (写実主義), which focuses on depicting reality. The sentence is followed by 「事実は良材なり。されどこれを役することは、空想の力によりて做し得べきのみ。」, which in essence means "Although reality provides a good source for story-writing, it is only through the power of imagination that such reality can be put into good use."

Although 事実を得て literally means obtaining facts, I believe what it means is "depicting facts" because it is all about writing. Since 天来の妙想 is contrasted with factual things and is followed by 空想の力, I would translate it as one's own unique idea. It seems to refer to "brain work" as opposed to facts. "God-given (or awesome) idea" doesn't fit well here.

The original essay by Ogai is as below. (See the last paragraph.)
http://shimohara.net/nitona/bungaku/ogai.htm