Well, my advice is: forget N-Gage...
Slow processor, too little memory, inadequate screen resolution... Additionaly Symbian (which would probably mean a lot of work to adapt it anyway...).
Some months ago when I was thinking about what kind of new phone I should get, I spent a lot of time by looking for a Symbian DOS emulator (I was satisfied with my Nokia 6230 (not smartphone), so I thougth I would buy another phone - this time smartphone - from Nokia). Nothing... And it looks like not easy task to do. There are promises of "PocketDOS" port for Symbian, but they have been promising it for years... I do not know about any real attempt to port DOSBOX to Symbian (and I was searching the Internet for long time). So I had to abandon Nokia and to go elsewhere...
Even if anybody would do that, it would be for S60v3 (QVGA display, more power and the platform is being further developed).
In short: if you want a DOS emulator (PocketDOS, DOSBOX) on a phone buy something with Windows Mobile (PocketDOS is even for "Windows Mobile smartphone edition", DOSBOX only for "Phone Edition" - the one with touchscreen).
Alternatively you can try to port it, though I do not see any sense in that. I have MDA Vario (HTC Wizard) which is far more suited to DOSBOX than N-Gage (MDA Vario has "Windows Mobile 5 Phone Edition", QVGA touchscreen, 64 MB RAM, 195 MHz processor, QWERTY hardware keyboard). From my experiences - it is usable for PC XT games !
Thinking about N-Gage with 104 Mhz processor, small screen, small memory - even if you managed to do this it would be just a demonstration that it works, not usable for anything...
You want the power 16 Mhz 80286 VGA computer... Well, I am *unable* to get even close to this on far better hardware (MDA Vario)... It is an absolute nonsense to expect anything like that on N-Gage...
Saying that, if you want to do port, do it it for Symbian S60v3. That platform looks better. And I think many people would like it... Well, there was a time when I looked for this port deperately... - before realizing that only "Windows Mobile" is an alternative today (well, in future there may be some Linux based phones - but they are rare at the moment and I have never seen any myself and I certainly would not get one subsidized from my mobile phone operator)...
(Well there is also a port for Palm made by somebody last year - but from what I know it is nowhere near the real usability yet and since it is made by one man team it may never get there).
Mirek