How big is ur budget? Are u looking to race or are u more of a basher? There's alot of motors out there bro, we can help u choose the right one that fits ur needs but ur gona have to give us a little more info on what ur looking for.
I do NOT recommend ofna motors. The reason I do have an lrp is because on day 33 after purchase of an ofna force 28, the engine had obvious mechanical malfunction and slung the rod and tore up the piston, cylinder, and internal crankcase. Ofna said they would not replace due to it being 3 days after their 30 day warranty. I'm not a professional but I do know how to follow directions very well and I know it was nothing by my doing.
I been running an ofna force .28 in my buggy for 6 months now and I've never had an issue with it. Matter of fact it runs stronger and better than the F4.6 i had in it before that. Also yes your exhaust and filter will work on the new engine you choose.
Force .28 is the same motor as the F4.6, Force makes that motor for HPI. U can find a F4.6 on ebay for dirt cheap from various chop shops. Not sure what they go for but I sold mine on for $105 brand new removed from a RTR.
IMO just spend a little bit more and you'll enjoy your savage a lot more. I'm on tank 16 on the lrp and man she's becoming a monster. Wheelies on demand, it's an XL, great low end and top end. Break in was much easier, even when the lrp came in the mail just how it's packaged shows what kind of company they are, every hole had a rubber cover. FYI, I also have an ofna ultra lx comp 1/8 buggy that's old but never had any problems.
Agree with Americandrug 100%. If it's within ur budget then by all means please do ur self a favor and pick up an aftermarket mill, u will be alot happier knowing u spent a couple extra bucks on a higher quality engine.
I suggest u look into these two, u said ur looking for something "not too expensive" I picked up each one for less then $200...