Proving $L = \{\langle M, w, n \rangle$ : $M$ accepts $w$ within $n$ steps $\}$ is decidable
This is not a problem. You have an infinite tape, so you can store whatever you want on it.
Depending on how explicit your proof should be, consider a few edge cases for defining your transition function:

a.) How does your TM recognize, if all n 1s are gone? The complexity of this depends on the alphabet of the TM, if $|\Sigma| &gt; 2$ it should be easy, else you have to work out a few details. 

b.) After you've marked one of the 1s - how do you get back to the next step of your computation?