6502 and the 8088/86 are going to be somewhat similar, both are CISC, etc. x86 has more instructions but like the 6502 just variations on a theme.
If you are talking about more than just a DOS emulator something like an 80386 or newer then yes that is a much bigger effort. Not only the processor, but also the system. A 6502 based game system doesnt have too much around it, often specific to the platform, asteroids, nes, c64, etc. Dos you are going to need to come up with a bios and then a dos, which I believe at this point are out there for free. but also the peripherals that many 8086 systems have/had, video, hard disk, etc.
Many dos apps would talk to hardware directly so you have to do more than just emulate the bios calls.
Add all of this up and the level of effort for a dos/x86 system is much more than a 6502.
You dont have to create an emulator that is design for efficiency, not anymore (for dos for example). The sources you are looking at (MAME is a good source for a bunch of processor simulators) were designed for speed most likely and many are barely readable.
x86 is a painful instruction set from any direction you look at it. If 6502 is all you have done so far I would choose something else next, unless it is obvious how to do it (which it sounds like it isnt). maybe a fixed length instruction set like arm or mips (well the fixed length versions). Or to get a sense of the scale, do a z80 and run space invaders or galaga or something on it. (hmm is space invaders an 8080?) (a z80 derivative is in the gameboy).