I wouldn't advice this, you can see other questions on here about people asking if they can use different model numbers on their phone and everyone says no.
A suggestion to you would be the following, go on the XDA website here link and type in your phone AND model number into their search bar. From there you should be able to click on their forum for Android Development, from there you can select a Custom Rom to flash to your phone
As you said you are new it is required to firstly root your phone before flashing a new ROM, as you haven't provided your Phone Model I cannot give you a direct link on how to root your phone, if you can update this detail I could find you a link afterwards.
Also if you decide to root your phone flash a custom rom please note that it is your risk, doing so will void your warranty and if you end up with a bricked device(not working) I or anyone giving instructions in any links you use cannot be help responsible.
With regards to the problem with only Sending a SMS if it's 80 Char or less, I am not 100% sure as I haven't experimented with various different Custom ROMS but there may be a option that you can select to try and fix it, personally I would post another Question on this website just for that issue
All the best