Spectron was really good in its day, but Class D has come even farther than Spectron was pushing. Many of the diy amp kits will net you good results, but if you can afford to invest more, there are some excellent Class D amps on the market from notable manufacturers now.
i would not reuse Spectron’s parts, either. If you don’t know them inside and out. Generally the transformers, power supplies etc are all designed around the voltages and performance parameters of that amp particularly. They won’t work with just any Class D Board. And given their age, I wouldn’t want to risk the safety of new components.
I’m not sure what speakers you are trying to power, but as a T+A authorized dealer, I have a T+A A 200 amplifier that uses Purifi, and it is very good. T+A also now makes monoblocks as well for quieter operation and increased power output. Aavik also makes stellar Purifi amps. I have no affiliation with them but they do push the boundaries for Class D in ways Spectron was doing so many years ago.