Strictly speaking (or at least traditionally), the singular of data is datum. However, it's increasingly frequent that people simply use data in both singular and plural forms.
This is a strange word. When used in the singular, using the indefinite article is fine and the verb should be is. When used in the plural, there is no article and the verb is are. However, you also have the option of using data as an uncountable mass noun, with is and an optional definite article.
I would also say data from (or data collected from) rather than data of. And because of that, I would probably not use from a second time in the sentence.
1. A datum / data from 300 observations is generated by using a normal distribution.
2. a) Data from 300 observations are generated by using a normal distribution.
2. b) (The) Data from 300 observations is generated by using a normal distribution.
This is one of those rare situations where either is or are can be used (in the last two versions) and both are actually correct.
It seems to me that unless you're talking about Xs of data or data Xs (points of data or data points, for instance), it's simpler to use the mass noun form—and forget about it being singular or plural and which word to use. However, I know that many scientific texts still follow the singular and plural forms.
What makes this tricky is that different people use it differently, and it's easy to confuse the different uses.
Even odder is that, in referring to Merriam-Webster on this, I discovered that the plural of datum can also be datums. (Something I hadn't known.) This makes things even more confusing.
Here is what Merriam-Webster says about the the word:
Is data singular or plural?: Usage Guide
Data leads a life of its own quite independent of datum, of which it was originally the plural. It occurs in two constructions: as a plural noun (like earnings), taking a plural verb and plural modifiers (such as these, many, a few) but not cardinal numbers, and serving as a referent for plural pronouns (such as they, them); and as an abstract mass noun (like information), taking a singular verb and singular modifiers (such as this, much, little), and being referred to by a singular pronoun (it). Both constructions are standard. The plural construction is more common in print, evidently because the house style of several publishers mandates it.