Onan generator isolators available from Cummins dealers, RV parts suppliers, or industrial suppliers like Grainger. Measure bolt size and diameter to match if using generic mounts.
Onan 4KY that starts but stops when start switch released is usually oil pressure sensor failure. Check oil level first. Sensor can be bypassed temporarily by grounding wire but verify actual oil pressure.
Used Onan 4000 generators range $800-2500 based on hours and condition. Check hour meter, run under load, and inspect for carburetor issues before buying.