G0MRF has produced a design and PCB for a 300W class D PA for the MF band. It needs a low level drive at twice frequency. For WSPR it needs the experimental software from K1JT that produces tones at twice frequency so they are right when driving this PA. Although kits of some parts are available, I do not think a complete kit is available. Using such a PA would increase my 630m ERP by several "S" points, even without antenna and ground changes.