Coils icing up. Any way to fix?

[wm69]

I have a Rapids kegerator (they quit making them @20 years ago so it's definately an older model). Works fine except that since it's gotten hot outside (it's in the garage) I've got ice buildup on the coils which is raising the temp of the beer. It's still cold, but not as cold as I'd like it to be. I assume that this is manual defrosting type and was wondering if there is an inexpensive way to add a defrosting heater/auto timer to keep the ice buildup to a minumum. I didn't have a problem with this until the weather got hot. Could it be an over/under charge of freon?

I've had it since around Jan or so. I did just recently replace the condensor fan as the bearing went out on the old one. The old one was a pull fan and I moved the brackets around as the only one I could find that would fit was a push fan. I flipped the bracket around and it worked fine until it got hot in the garage, which is when the ice accumulation started. I cleaned the coils well when I installed the new fan/ POR 15's the inside of the cabinet. Should I attempt to fab up a fan shroud of some kind to attempt to direct more air onto the coils? I know it's not unusual for older fridges to need defrosting and was thinking I might be able to add a coil heater/timer like newer self defrosting refridgerators have.