Well Shaun gave you the pic - a painting by Stan Stokes - of Rickenbacker in his Spad.
A quick zoom around on google last night - before my pc spat the dummy so I couldn't post this - shows that 94 "Hat in the Ring" squadron received Spads from 5 July 1918 and was all Spads by 8 August 1918. Rickenbacker flew Spads from 5 July so this victory must be after that date.
All his victories over single seaters while he was in A Spad were over Fokkers.
Assuming your research of the Fokker's colours for your diorama are correct - (its just that the blue isn't obvious in the painting) and I have no doubt that it is correct, then the Fokker is from Richtofen's Circus (red nose) and the blue fuselage with white tail indicates it is from the 10th Pursuit Flight of the circus.
As the pilot was obviously not killed a search of german air casualties won't assist in identifying the German pilot.
Also I read Rickenbacker's journal online and could not fine reference to him downing a Fokker that landed in a creek/puddle/swamp. Many he described as going down in flames or exploding on impact with the ground. There was one that he was trying to capture that another Spad interrupted and shot down - thjis one crashed near a wood and he in fact drove near that site the next day when he went for a drive to visit his brother in the trenches.
The possible dates for the victory are 8 or 10 August 1918; 14,15,25,26,28 September 1918; or 1,2,3,9,10,22,23,27 or 30 October 1918. It is definitely not his last victory - which was over a balloon.
My guess is it relates to his victory on 25 September 1918 - for which he was awarded the Medal of Honour and the DSC w/7 OLC, where he shot down one of a number of escorting Fokkers and shot down a Halbetstadt 2 seat recce plane. It is also the first time that he scored a "double" in a day. If this is the case then it is "official" victory number 9 of his 26 "official" victories.
This is a great thread - but its not helping me get a few models finished for our annual Club comp in May.
Brian