I'm not really sure what you're asking. Certainly for any natural number $n$, the structure $\mathbb{N}_n$ consisting of the natural numbers up to $n$ (with $+$ and $\cdot$ interpreted *relationally* in order to be a legitimate structure) is trivially decidable, so in that sense Godel doesn't apply to it(s theory).

But there's a huge issue here: checking whether a sentence of length $<n$ is true in $\mathbb{N}_n$ requires more than $n$ steps in general. So the decidability of $\mathbb{N}_n$ is not satisfying from an ultrafinitist standpoint, since the completeness itself isn't "justifiable within $\mathbb{N}_n$." Meanwhile, the non-ultrafinitist won't be impressed either since $\mathbb{N}_n$ is too limited a structure to treat even basic arithmetic. So this dodge doesn't seem satisfying from any perspective.

It's worth noting that Godel's *second* incompleteness theorem can be avoided non-trivially with a trick of arguably ultrafinitist spirit: there are theories which interpret a decent amount of arithmetic but which prove their own consistency, and a key component of such theories is that they do not prove that multiplication is total, so in a weak sense have ultrafinitistic flavor. These were first studied by Dan Willard.^{1}

However, Godel's first incompleteness theorem still applies to these. Indeed, no theory which can prove each true quantifier-free sentence about $\mathbb{N}$ and, for each $k\in\mathbb{N}$, can prove $$Init_k:\quad\forall x(\underline{k}<x\vee\bigvee_{0\le i\le k}x=\underline{i})$$ *(where "$\underline{m}$" denotes the numeral corresponding to $m$)* is going to be complete, consistent, and computably axiomatizable (see e.g this paper of Ritter). Note that such theories are not required to prove that multiplication is total, or that $<$ is a linear ordering of the universe, or so on: they are truly quite weak.

^{1}Dan Willard: Self-Verifying Axiom Systems, the Incompleteness Theorem and Related Reflection Principles, Journal of Symbolic Logic 66 (2):536-596 (2001). DOI: 10.2307/2695030, JSTOR, author's website