Translating sentence of Luxun Lu Xun says here: 


  夫國民發展，功雖有在於懷古，然其懷也，思理朗然，如鑒明鏡，時時上征，時時反顧…


夫 is probably the literary auxiliary for introducing a new topic. 

有，I assume means “partly”

思理：I’m very unsure about this. Might go with 思辨能力: the ability for analytical thought
朗然:brightly, clearly?

My translation attempt:


  As for the development of the populace, the merit in part depends on keeping the Ancient Tradition. But it’s keeping must happen with clear analytical thought, as a mirror or a clear lens, at times proceeding, at times looking back...


There are many parts about which I am uncertain. Can you point out mistakes or improvements?
I think your translation is mostly accurate. But I don't think 有 means "partly" directly. In Chinese, 有 usually means "there is/are". So a more "verbatim" translation would be

..., although there are merits in keeping the ancient tradition,...

with the following correspondence

虽=although

有=there are

功=merits

在于=in

怀古=keeping…

As for 思理, you may know that 思 means "think, thought", and 理 means "reason". So 思理朗然 means

with clear thinking and reasoning

or

the thinking and reasoning must be clear