You're better off getting a new bike. The old BMX bikes really are pretty small. Even a $300 complete these days is about 2" roomier in the Top Tube than many OS bikes. There have been so many improvements made in BMX bikes the past 25 years, that riding an old bike kinda scares me at times. I'll take my new BMX bikes over the old ones for park riding anyday.
I'm 40, 190lbs BTW
-Bill