My core is imo better than David's:
- It has a builtin software serial implementation on the AIN0/1 pins (uses the analog comparator interrupt instead of a pcint, so it interferes with fewer other things than most software serial implementations).
- It has configuration options for almost every conceivable clock source and fuse setting, and supports almost every attiny device.
- It has my "unified wire" and "unified spi" libraries, which allow you to treat any of the attiny devices my core supports as if they had real hardware I2C or SPI, and it will pick the appropriate implementation to use for that hardware and present a compatible interface, so code and libraries can usually be used without modification.
- It is actively and aggressively maintained.
Anyway... why not leave timer0 alone and use timer1 to generate the PWM?
It's perfectly capable of generating PWM, in fact, better at it than timer0 (timer1 has every power of 2 as an option in the prescaler, and you can clock it off the internal PLL giving it a base frequency of ~64mhz if you for some reason need very high frequency PWM)