The bike itself is a small frame model that a company called Bird produced and sold. They were sold with various names and small changes through the years, but they made them from 1969-about 1985 yours is a later version due to the steel scrub brake pad (that hits the tire) and the center kickstand, problaby even post 1980 since there is not a fork plate. Heres an '83/'84 I did a "spraybomb refresh" years back as an example to compare to your frame...
On the engine, well its not originally a mini bike engine. can still most likely be used of course with some mods and it is in the same HP range most likely as the original. I If it does still retain its tag with the engine numbers, I would be interested to know what they are......there is something, rather lack of something, on that block and has me curious about its setup.
Have fun with it, those are small bikes but have a really cool classic minibike look to them!