As a Maryland lawyer, I cannot comment on Pennsylvania Law. However, most written contracts will be enforceable, as long as the terms are legal. In a situation like this, a contract and promissory note signed by the individuals involved could work. The note should contain the essential terms such as the amount of the loan, the interest rate, and the terms of repayment (when installments are due and when the note is due and payable). The note would most likely be unsecured by any collateral, so the issue is whether or not you'd be likely to collect the debt if the contract is breached. Be sure to seek the advice of a competent lawyer to ensure what you are doing is enforceable and/or collectible.
The answer provided above is for general informational purposes only and is not intended to create and attorney-client relationship. Please seek the advice of competent counsel licensed to practice in the appropriate jurisdiction if you so desire.