The only commonly available drop in that is pretty bright, as Pounder stated, is the Terralux TLE-300M-EX that uses three Cree XR-E LEDs. I have two of them, one in a 4 D Maglite and another one in a 2 D Maglite powered by a 6 AA battery holder. My 4 D one was tested in Bigchelis' sphere at about 608 lumens at initial turn on. (Terralux rates it at 700 lumens.) It is brighter than my Fenix TK40 initially, but after three minutes its output dropped down to about 473 lumens. I don't know if the output dropping was due to the lousy alkaline batteries or heat build up. I'm curious what my new one will do as it is powered by Duraloop/Eneloop AA batteries.
I'm not aware of a drop in that is currently available and puts out at least 1000 lumens. Maglite drop ins are all pretty much a trade off between brightness, run time, and heat build up. I've seen some super bright modified Maglites, but they usually don't have any run time because the LED(s) are so over driven. Then they usually don't have very good heat dissipation so their brightness drops rapidly.
The TLE-300M-EX is a pretty good compromise between the three trade offs. To help with the heat dissipation, I used Arctic Silver 5 on both of mine to help the heat transfer from their heat sinks to the aluminum Maglite body tubes and heads. The TLE-300M-EX does have three output modes though, but the previous TLE-300M model only has one like you wanted. The older model is factory rated by Terralux to put out 600 lumens but I don't know what it really puts out the front. It's easy to tell the difference between the two as the TLE-300M has a black front face and the TLE-300M-EX has a gray front face.
The cheapest place that I could find the TLE-300M-EX at was Light Hound:
http://www.lighthound.com/TerraLUX-TLE-300-700-Lumen-Upgrade-for-4-6-D-Cell-MagLites_p_2150.html
Enter in the coupon code "cpf" to save an extra 2%.
Before someone mentions it, there also is the Malkoff Devices drop in upgrade for Maglites that is very highly regarded by other CPF members and costs $69. It is almost always out of stock though (thus I have never used one) plus at 164 lumens at initial turn on with 4 D alkaline batteries, it doesn't come anywhere near your requirement of 1000 lumens. That output drops down to 139 lumens after 30 seconds. Malkoff Devices rates it at 260 lumens. It's shown here:
http://www.malkoffdevices.com/shop/...ite-p-1.html?zenid=u61dsl14v91mdgkv7gr4dh6f42
The test results of these drop in upgrades are in this thread:
http://www.candlepowerforums.com/vb/showthread.php?t=260659
