I am building a Bruce Tharpe Venture 60 and have come across a small issue. I am mounting the engine mount to the firewall and noticed the mount is a little off center from my original markings. Just to clear any possible confusion on the F1 former (firewall) there is a vertical “center line “ and a horizontal “thrust line” the mount ended up perfect on the thrust line but just a hare off center to the left on the center line. My Q is can I keep building and disregard this small error or is this going to cause a problem with the flight or my Venture?

Depends, how "small" of an error?

You could email Bruce <mailto:[email protected]> with as much info as possible for the best possible answer.
He'll probably want to know an actual measurement rather than "just a hare off center to the left on the center line."

I am not sure what you mean by "slightly off" but if I were mounting a engine mount I would want it within 1/32" of a inch form the dimensions called on the plans. So if you are that close already you did good INMO. If you are off more than that you can increase the dia of the holes in your engine mount to the next size. Bolts holes should be 1/32 " larger than the the dia of the bolt this seems like a lot but this actually the standard in manufacturing. If that does not get you on the centerline you will have to slot the holes in your mount with a dremel tool. Yes you can keep building you will just have to modify the motor mount slightly.