Post some pics. Let's see your ride. You've come to the right place for advice, help, and self-imposed harassment. JK.
You're right that you want to get your car reliable as 35+ year old cars need a lot of TLC to get them to the point where you're not working on them as much as you drive them.
About more HP, there are lots of engines and engine builds to get the job done. Most require lots of money and most require lots of engine work, either just on the engine (T4 big bore, etc.) or modifying the chassis to accept an engine not designed for the 914 (V6, V8, Subaru -4 or -6, or Porsche /6).
Aftermarket devices like exhaust, cold air intake have minimal effect on the engine. A 75HP engine isn't going to jump to 100+ HP with some simple mods. Just get your car running good then look into the alternatives. 914's were never the fastest cars but they handle better than most.
One warning though about engine rebuild or swap - when you rebuild your engine or install some other engine, your car will likely be down for a long period of time (months to years), cost much more money than you originally planned, and hopefully you enjoy working on your car as much as you like driving your car.