I highly suggest you increase your budget to the 40K range or wait till you can and then swap this engine out for the EFi D15b the normal and common one or the one I would recommend is the D15b 3 Stage Vtec one, You'd get pretty decent power and pretty good mileage figures on both the normal fuel (CNG & Petrol) with the 3 Stage Vtec one.
You can try to salvage the current engine and spend more on rebuilding it but I would strongly advise to not do that, I had the same problems with the same engine in my Old Civic and I kept on pushing the same engine and putting more money into it but it would hardly give me a few days before something new or an old problem showed it's head again and I got so fed up with it that I ended up selling the car pretty much in a state when it wasn't really driveable anymore. The CNG mileage suffered because of that and the petrol mileage was worse than a tank.
So taking from my personal experience with a faulty engine, I'd highly suggest you opt out for a swap and therefore save yourself from the headaches and regret, You can get a decent deal for a new engine in exchange for your own, You should go out to the market on a free day and do a bit of "window shopping" to see what's what,
I have the 3 Stage Vtec D15b in my current car and believe me I would never want to go back to the everyday problems and headaches of the old carburetor D15b3. The newer engines makes your life so much easier or at least it has for me.
I hope this can you help you in making a decision for the car.