No way are AA (or any) dry cells practical, they have a high internal resistance and just wouldn't put out the Amps.
Rechargeable AA would work (for a short while). Heres a quick example..
I found 2900mAh NiMh AA on ebay; highest capacity AA that I could find. That is the same as 2.9Ah (just convention with small cells to rate in mAh)
As Ratfink calculated 35W at 12V is just about 2.9A
So a pack of 10 (1.2V each cell) would theoretically give you 12v for an hour.
Although the 2900mAh rating will probably be at 1/10 capacity draw, so reckon on a fair bit less than an hour.
I would guess at somwhere around 45 mins. This would depend on the quality of the cells.
Sealed lead acid (SLA) batteries have higher capacities but are clunky. You could go for bigger NiMh's a string of C or D cells could be dressed up as an extra frame tube :lol:
LiPo cells have a higher energy density (more electric for the size/weight) but have big drawbacks on charging and discharging. I am a radio ham and run LiPo's in my RC but they won't be anywhere near my ride :wink:
I am with you on wanting to keep the tractor light, they are cool and I have my eye open for an old one at the right price. WW2 saw my area stripped of any metal 'to melt down for spitfires' the home counties around London were right behind the war effort, so you won't find many 'treasures' around here
The suggestion of converting to LED is the best way to go if you can manage it. Higher efficiency = More light for less current.
Regards, Kev