1250 ft is not very much. If you feel the bike lacking some zing, or steep/long hills normally where you ride, I'd go for 20 but the 45 should pull 21 with a bit more shifting gears. But I let my engine rev quite hard. Not all like to ride like I do. 19 is sidecar gearing but might be needed if very steep hills or slow mean speed is normal in your area. 

This is taken from the 1929 Indian notebook. Roads where a bit different then, narrower with more hills and more dirt roads so a bit slower mean speed was the norm. With hard inflated tires on roads of today, a tooth or two less on the rear sprocket should not be a problem in most cases.

Standard factory gearing Indian Scout 37ci:

Solo Standard: 21-40

Solo Heavy Duty: 20-40

Sidecar Standard: 18-40

Standard factory gearing Indian Scout 45ci:

Solo Standard: 21-36

Solo Heavy Duty: 22-40

Sidecar Standard: 19-40

Standard factory gearing Indian Chief 61ci:

Solo Standard: 21-40

Solo Heavy Duty: 20-40

Sidecar Standard: 19-40

Sidecar Heavy Duty: 18-40

Standard factory gearing Indian Chief 74ci:

Solo Standard: 22-40

Solo Heavy Duty: 21-40

Sidecar Standard: 19-40

Sidecar Heavy Duty: 18-40

Standard factory gearing Indian Four:

Solo Standard: 17-40

Sidecar Standard: 15-40

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It is easy to make a grid chart with different numbers to get the feel of different gearing. Just divide the numbers with each other. 

36/21=1,714 and 40/22=1,818 as an example.

That will show that 36-21 is about similar to 38-22 or 34-20 gearing.

40/22 is about the same as 36/19, but a small front sprocket wears more on the chain and sprockets.

I found that 19" wheels equals exact one cog less. 19" wheels with 36 teeth sprocket = 18" wheels with 35 teeth.