Looks like a nice design by S&S. South Hants Engineering built them, hence the “SHE”. Blowing up an image of the plans shown here on Sailboatdata might give an indication of the original engine installed, or its horsepower. Barring that you might want to measure the engine mounts and available space and then talk to a mechanic. A quarter-tonner is not going to need a super-powered engine, and there may be many suitable. They will not be cheap.
Repairing the shroud anchor points will depend on what the problem with them is. A photo would help better define what’s wrong. On a boat of this period the shrouds likely attach to the main bulkhead. If it is rotted or delaminated it will need to be fixed – a major repair. If the shrouds attach to flanges that are fiberglassed to the hull, the fix may be somewhat less complicated. Good luck!