Q:

How can I modify this MOV assembly instruction to write a hard coded value?

I have the following line of x86 assembly that I'm trying to 
mov     byte ptr [esp+1F4h+var_4], bl

Encoded as 
88 9C 24 F0 01 00 00

...which I would like to modify to write a hard coded byte rather than the bl register. ...but I'm having trouble reading x86 the op code table here in order to modify this byte code.  Also, are the two trailing null bytes to delimit the byte code from the next instruction, or can they be overwritten (in case the new instruction is longer)?

A:

When in doubt just use an assembler ;)

the two trailing null bytes to delimit the byte code from the next
  instruction

Those are not delimiters, they are part of the offset which is stored as 32 bits. The new instruction should be the same length. It's hard to fit a longer one, but a shorter one can be padded with NOP instructions. Unfortunately the version with the constant is indeed longer:
C6 84 24 F0 01 00 00 42 mov byte [esp+0x1f0], 0x42

You can't fit this in there, you might have to mess with the following instructions too, in case you can shorten them to make room.

