Did you do what Kyle said to?
I just patched Kung Fu Kid with Infinite Health (084-D4E-C42), and it worked perfectly fine [on an emulator].

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Edit:
While the patching does work fine, I found out that the particular code I made will also make the boss have Infinite Health.
If you want Infinite Health in Kung Fu Kid, you'll need to enable ALL of the following codes:
Kung Fu Kid (USA, Europe)
Infinite Health (Bosses Too)
084-D4E-C42
Progress To The Next Stage Even With Infinite Health Enabled
18D-B9F-4CA
One Hit Kills Boss (Stage 1)
18C-F4D-4CA
One Hit Kills Boss (Stage 2)
182-77C-4CA
One Hit Kills Boss (Stage 3)
009-07C-C4E
Able To Kill Bosses (Stage 4)
642-628-808
722-638-6EA
Use Down+2 Button.
Use Down+2 Button to kill the boss on stage 5 also.
One Hit Kills Initial Boss (Stage 6)
00C-4AB-6EE
Able To Kill Next Bosses (Stage 6)
001-6EA-E69
One Hit Kills 3rd Boss (Stage 6)
00E-E0B-4C2
One Hit Kills 4th Boss (Stage 6)
189-A5B-4CA
One Hit Kills 5th Boss (Stage 6)
185-B6B-4CA
One Hit Kills Final Boss Of the Game
EF6-ED8-A26
766-EE8-3B7
I was able to beat the game using all of those codes (and Infinite Time).