I am trying to figure out where I can get more tickets to increase my chances of getting invites. For now, I only get a ticket for a daily marathon, but those are 3-4 stars and no invites.
You are focusing way too much on getting invitations. The original advice given to you was that IF you receive an invite, it would be best to use it to purchase the car(s) because invites can be very hard to get. You have been told many times that currently the only way to get invites is from the daily marathon each day. There is no other way to increase your chances. Keep playing and the invites will come at some time. Unfortunately, you can receive duplicates of invites that you have already received and used before you get new ones.I now see that I need to be collector level 50 to be able to do it. I am 43. Probably some week/months to go. Also, I have civic 98 not 93. Also ok?
You do not need to be at collector level 50 to do engine swaps with the engines you have in the tuning parts section of the garage.
If you want to try doing a swap, I suggest using the Toyota 86 GT '15 from Brand Central for only 29K. You might already own one. As suggested, it would be good to own 2 of them so you can have one stock and one to do the K24A-Civic engine swap. This car has good handling with the stock default tune settings. I would then add sport hard tires to both cars and use both of them in the current weekly challenge Clubman 550 event to compare the difference in performance between the stock engine car and the swap engine car. I think it would be good for you to experience this part of the game. This car can also be used in quite a few of the races.