Depends on what capabilities on working the stuff you have.
Can you weld it? If yes, you want grade 2 or 3.
If no, you are limited to bolting or rivetting it to a chassis/frame. Then the higher grades are fun.

Weigth for weight, titanium is twice as heavy as ali, and 2/3ths the weight of steel/hardox.
That gives you, if you just replace the ali, without change in weight, 1.5mm Ti.