Hi, it would be perfectly acceptable for you to write the Will, provided it is signed and witnessed correctly. With home made Wills, the Probate Registry often ask for one or both of the witnesses to sign a declaration that the Will had been executed correctly.
You of course could not act as a witness, and nor should anyone who is going to benefit from the Will or any beneficiaries spouse.
Under no circumstances should the signed and executed Will be altered. I am afraid you will need to prepare a new Will.
Provided the current wording makes it clear to anyone reading it, that will be fine!