SPA = senior pattern association
http://www.rcpattern.net/spa/
Here's what it says from the website about altering planes:
a: Any model designed and flown before January 1, 1976 is legal
b. Aircraft must be built to the plan form of the plans.
c. Retractable landing gear, flaps etc. are not legal.
d. The contestant is responsible to furnish documentation for any model not on the Legal Airplane List. The president can provide the member with Legal Airplane information.
Best regards,
Scott