I think the noun tumbo can be translated as belly, as well as stomach, abdomen and womb, and this is also the plural for utumbo (intestine, entrails) but then the u class agreement is needed (so I think the translation for Big/large/huge intestines would be Tumbo nkubwa but perhaps only the singular form Utumbo mkubwa is used.) I really do not know if there is other Swahili word for belly.
There is no nkubwa. The N class prefix only occurs on adjectives and U/N nouns where it can make a prenasalised consonant and the only prenasalised consonants are:
mb (from b, w) (eg. mbavu, mbili)
nd (from d, l, r) (eg. ndogo, ndimi, ndefu)
nj (from j) (eg. ?)
ng (from g) (eg. ngumu)
mv (from v) (eg. ? ... I think it this "m" is sometimes absent)
nz (from z) (eg. nzuri)
It also appears as ny before a vowel (eg. nyingi, nyuso)
Otherwise, there is simply no prefix ... with the exception of words which would otherwise have to be one syllable, such as -pya, which takes a syllabic m-.
I have trouble to find so many parts of the body also using my dictionaries. Is it possible don't exist names for common things like : heel, muscle, eyelid etc ?
musuli or msuli = muscle (plural misuli)
tafu = muscle (plural tafu)
kisigino = heel (plural visigino)
ukope = eyelid/eyelash (plural kope)
You can try www.glosbe.com/en/sw/
I assume your native language is Portuguese, so you could also try https://glosbe.com/pt/sw/ ... even if it doesn't have an entry, there's quite a lot of translation data down the bottom.
If Glosbe doesn't have something, you can try http://www.elimuyetu.co.tz/subjects/arts/swa-eng/ - Swahili to English http://www.elimuyetu.co.tz/subjects/arts/swa-eng/ - English to Swahili ... but the page for words starting with "s" is missing :-/
This seems to be mostly from the same source, the TUKI dictionary http://www.swahili.it/glossword/index.php?a=index&d=8